diff options
Diffstat (limited to 'library/std/src/sync')
23 files changed, 2695 insertions, 2826 deletions
diff --git a/library/std/src/sync/mod.rs b/library/std/src/sync/mod.rs index 7b507a169b3..4fee8d3e92f 100644 --- a/library/std/src/sync/mod.rs +++ b/library/std/src/sync/mod.rs @@ -182,6 +182,7 @@ pub mod mpsc; mod barrier; mod condvar; mod lazy_lock; +mod mpmc; mod mutex; mod once; mod once_lock; diff --git a/library/std/src/sync/mpmc/array.rs b/library/std/src/sync/mpmc/array.rs new file mode 100644 index 00000000000..4db7b4990b9 --- /dev/null +++ b/library/std/src/sync/mpmc/array.rs @@ -0,0 +1,513 @@ +//! Bounded channel based on a preallocated array. +//! +//! This flavor has a fixed, positive capacity. +//! +//! The implementation is based on Dmitry Vyukov's bounded MPMC queue. +//! +//! Source: +//! - <http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue> +//! - <https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub> + +use super::context::Context; +use super::error::*; +use super::select::{Operation, Selected, Token}; +use super::utils::{Backoff, CachePadded}; +use super::waker::SyncWaker; + +use crate::cell::UnsafeCell; +use crate::mem::MaybeUninit; +use crate::ptr; +use crate::sync::atomic::{self, AtomicUsize, Ordering}; +use crate::time::Instant; + +/// A slot in a channel. +struct Slot<T> { + /// The current stamp. + stamp: AtomicUsize, + + /// The message in this slot. + msg: UnsafeCell<MaybeUninit<T>>, +} + +/// The token type for the array flavor. +#[derive(Debug)] +pub(crate) struct ArrayToken { + /// Slot to read from or write to. + slot: *const u8, + + /// Stamp to store into the slot after reading or writing. + stamp: usize, +} + +impl Default for ArrayToken { + #[inline] + fn default() -> Self { + ArrayToken { slot: ptr::null(), stamp: 0 } + } +} + +/// Bounded channel based on a preallocated array. +pub(crate) struct Channel<T> { + /// The head of the channel. + /// + /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but + /// packed into a single `usize`. The lower bits represent the index, while the upper bits + /// represent the lap. The mark bit in the head is always zero. + /// + /// Messages are popped from the head of the channel. + head: CachePadded<AtomicUsize>, + + /// The tail of the channel. + /// + /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but + /// packed into a single `usize`. The lower bits represent the index, while the upper bits + /// represent the lap. The mark bit indicates that the channel is disconnected. + /// + /// Messages are pushed into the tail of the channel. + tail: CachePadded<AtomicUsize>, + + /// The buffer holding slots. + buffer: Box<[Slot<T>]>, + + /// The channel capacity. + cap: usize, + + /// A stamp with the value of `{ lap: 1, mark: 0, index: 0 }`. + one_lap: usize, + + /// If this bit is set in the tail, that means the channel is disconnected. + mark_bit: usize, + + /// Senders waiting while the channel is full. + senders: SyncWaker, + + /// Receivers waiting while the channel is empty and not disconnected. + receivers: SyncWaker, +} + +impl<T> Channel<T> { + /// Creates a bounded channel of capacity `cap`. + pub(crate) fn with_capacity(cap: usize) -> Self { + assert!(cap > 0, "capacity must be positive"); + + // Compute constants `mark_bit` and `one_lap`. + let mark_bit = (cap + 1).next_power_of_two(); + let one_lap = mark_bit * 2; + + // Head is initialized to `{ lap: 0, mark: 0, index: 0 }`. + let head = 0; + // Tail is initialized to `{ lap: 0, mark: 0, index: 0 }`. + let tail = 0; + + // Allocate a buffer of `cap` slots initialized + // with stamps. + let buffer: Box<[Slot<T>]> = (0..cap) + .map(|i| { + // Set the stamp to `{ lap: 0, mark: 0, index: i }`. + Slot { stamp: AtomicUsize::new(i), msg: UnsafeCell::new(MaybeUninit::uninit()) } + }) + .collect(); + + Channel { + buffer, + cap, + one_lap, + mark_bit, + head: CachePadded::new(AtomicUsize::new(head)), + tail: CachePadded::new(AtomicUsize::new(tail)), + senders: SyncWaker::new(), + receivers: SyncWaker::new(), + } + } + + /// Attempts to reserve a slot for sending a message. + fn start_send(&self, token: &mut Token) -> bool { + let backoff = Backoff::new(); + let mut tail = self.tail.load(Ordering::Relaxed); + + loop { + // Check if the channel is disconnected. + if tail & self.mark_bit != 0 { + token.array.slot = ptr::null(); + token.array.stamp = 0; + return true; + } + + // Deconstruct the tail. + let index = tail & (self.mark_bit - 1); + let lap = tail & !(self.one_lap - 1); + + // Inspect the corresponding slot. + debug_assert!(index < self.buffer.len()); + let slot = unsafe { self.buffer.get_unchecked(index) }; + let stamp = slot.stamp.load(Ordering::Acquire); + + // If the tail and the stamp match, we may attempt to push. + if tail == stamp { + let new_tail = if index + 1 < self.cap { + // Same lap, incremented index. + // Set to `{ lap: lap, mark: 0, index: index + 1 }`. + tail + 1 + } else { + // One lap forward, index wraps around to zero. + // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`. + lap.wrapping_add(self.one_lap) + }; + + // Try moving the tail. + match self.tail.compare_exchange_weak( + tail, + new_tail, + Ordering::SeqCst, + Ordering::Relaxed, + ) { + Ok(_) => { + // Prepare the token for the follow-up call to `write`. + token.array.slot = slot as *const Slot<T> as *const u8; + token.array.stamp = tail + 1; + return true; + } + Err(_) => { + backoff.spin(); + tail = self.tail.load(Ordering::Relaxed); + } + } + } else if stamp.wrapping_add(self.one_lap) == tail + 1 { + atomic::fence(Ordering::SeqCst); + let head = self.head.load(Ordering::Relaxed); + + // If the head lags one lap behind the tail as well... + if head.wrapping_add(self.one_lap) == tail { + // ...then the channel is full. + return false; + } + + backoff.spin(); + tail = self.tail.load(Ordering::Relaxed); + } else { + // Snooze because we need to wait for the stamp to get updated. + backoff.snooze(); + tail = self.tail.load(Ordering::Relaxed); + } + } + } + + /// Writes a message into the channel. + pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> { + // If there is no slot, the channel is disconnected. + if token.array.slot.is_null() { + return Err(msg); + } + + let slot: &Slot<T> = &*(token.array.slot as *const Slot<T>); + + // Write the message into the slot and update the stamp. + slot.msg.get().write(MaybeUninit::new(msg)); + slot.stamp.store(token.array.stamp, Ordering::Release); + + // Wake a sleeping receiver. + self.receivers.notify(); + Ok(()) + } + + /// Attempts to reserve a slot for receiving a message. + fn start_recv(&self, token: &mut Token) -> bool { + let backoff = Backoff::new(); + let mut head = self.head.load(Ordering::Relaxed); + + loop { + // Deconstruct the head. + let index = head & (self.mark_bit - 1); + let lap = head & !(self.one_lap - 1); + + // Inspect the corresponding slot. + debug_assert!(index < self.buffer.len()); + let slot = unsafe { self.buffer.get_unchecked(index) }; + let stamp = slot.stamp.load(Ordering::Acquire); + + // If the the stamp is ahead of the head by 1, we may attempt to pop. + if head + 1 == stamp { + let new = if index + 1 < self.cap { + // Same lap, incremented index. + // Set to `{ lap: lap, mark: 0, index: index + 1 }`. + head + 1 + } else { + // One lap forward, index wraps around to zero. + // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`. + lap.wrapping_add(self.one_lap) + }; + + // Try moving the head. + match self.head.compare_exchange_weak( + head, + new, + Ordering::SeqCst, + Ordering::Relaxed, + ) { + Ok(_) => { + // Prepare the token for the follow-up call to `read`. + token.array.slot = slot as *const Slot<T> as *const u8; + token.array.stamp = head.wrapping_add(self.one_lap); + return true; + } + Err(_) => { + backoff.spin(); + head = self.head.load(Ordering::Relaxed); + } + } + } else if stamp == head { + atomic::fence(Ordering::SeqCst); + let tail = self.tail.load(Ordering::Relaxed); + + // If the tail equals the head, that means the channel is empty. + if (tail & !self.mark_bit) == head { + // If the channel is disconnected... + if tail & self.mark_bit != 0 { + // ...then receive an error. + token.array.slot = ptr::null(); + token.array.stamp = 0; + return true; + } else { + // Otherwise, the receive operation is not ready. + return false; + } + } + + backoff.spin(); + head = self.head.load(Ordering::Relaxed); + } else { + // Snooze because we need to wait for the stamp to get updated. + backoff.snooze(); + head = self.head.load(Ordering::Relaxed); + } + } + } + + /// Reads a message from the channel. + pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> { + if token.array.slot.is_null() { + // The channel is disconnected. + return Err(()); + } + + let slot: &Slot<T> = &*(token.array.slot as *const Slot<T>); + + // Read the message from the slot and update the stamp. + let msg = slot.msg.get().read().assume_init(); + slot.stamp.store(token.array.stamp, Ordering::Release); + + // Wake a sleeping sender. + self.senders.notify(); + Ok(msg) + } + + /// Attempts to send a message into the channel. + pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> { + let token = &mut Token::default(); + if self.start_send(token) { + unsafe { self.write(token, msg).map_err(TrySendError::Disconnected) } + } else { + Err(TrySendError::Full(msg)) + } + } + + /// Sends a message into the channel. + pub(crate) fn send( + &self, + msg: T, + deadline: Option<Instant>, + ) -> Result<(), SendTimeoutError<T>> { + let token = &mut Token::default(); + loop { + // Try sending a message several times. + let backoff = Backoff::new(); + loop { + if self.start_send(token) { + let res = unsafe { self.write(token, msg) }; + return res.map_err(SendTimeoutError::Disconnected); + } + + if backoff.is_completed() { + break; + } else { + backoff.spin(); + } + } + + if let Some(d) = deadline { + if Instant::now() >= d { + return Err(SendTimeoutError::Timeout(msg)); + } + } + + Context::with(|cx| { + // Prepare for blocking until a receiver wakes us up. + let oper = Operation::hook(token); + self.senders.register(oper, cx); + + // Has the channel become ready just now? + if !self.is_full() || self.is_disconnected() { + let _ = cx.try_select(Selected::Aborted); + } + + // Block the current thread. + let sel = cx.wait_until(deadline); + + match sel { + Selected::Waiting => unreachable!(), + Selected::Aborted | Selected::Disconnected => { + self.senders.unregister(oper).unwrap(); + } + Selected::Operation(_) => {} + } + }); + } + } + + /// Attempts to receive a message without blocking. + pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> { + let token = &mut Token::default(); + + if self.start_recv(token) { + unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) } + } else { + Err(TryRecvError::Empty) + } + } + + /// Receives a message from the channel. + pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> { + let token = &mut Token::default(); + loop { + if self.start_recv(token) { + let res = unsafe { self.read(token) }; + return res.map_err(|_| RecvTimeoutError::Disconnected); + } + + if let Some(d) = deadline { + if Instant::now() >= d { + return Err(RecvTimeoutError::Timeout); + } + } + + Context::with(|cx| { + // Prepare for blocking until a sender wakes us up. + let oper = Operation::hook(token); + self.receivers.register(oper, cx); + + // Has the channel become ready just now? + if !self.is_empty() || self.is_disconnected() { + let _ = cx.try_select(Selected::Aborted); + } + + // Block the current thread. + let sel = cx.wait_until(deadline); + + match sel { + Selected::Waiting => unreachable!(), + Selected::Aborted | Selected::Disconnected => { + self.receivers.unregister(oper).unwrap(); + // If the channel was disconnected, we still have to check for remaining + // messages. + } + Selected::Operation(_) => {} + } + }); + } + } + + /// Returns the current number of messages inside the channel. + pub(crate) fn len(&self) -> usize { + loop { + // Load the tail, then load the head. + let tail = self.tail.load(Ordering::SeqCst); + let head = self.head.load(Ordering::SeqCst); + + // If the tail didn't change, we've got consistent values to work with. + if self.tail.load(Ordering::SeqCst) == tail { + let hix = head & (self.mark_bit - 1); + let tix = tail & (self.mark_bit - 1); + + return if hix < tix { + tix - hix + } else if hix > tix { + self.cap - hix + tix + } else if (tail & !self.mark_bit) == head { + 0 + } else { + self.cap + }; + } + } + } + + /// Returns the capacity of the channel. + #[allow(clippy::unnecessary_wraps)] // This is intentional. + pub(crate) fn capacity(&self) -> Option<usize> { + Some(self.cap) + } + + /// Disconnects the channel and wakes up all blocked senders and receivers. + /// + /// Returns `true` if this call disconnected the channel. + pub(crate) fn disconnect(&self) -> bool { + let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst); + + if tail & self.mark_bit == 0 { + self.senders.disconnect(); + self.receivers.disconnect(); + true + } else { + false + } + } + + /// Returns `true` if the channel is disconnected. + pub(crate) fn is_disconnected(&self) -> bool { + self.tail.load(Ordering::SeqCst) & self.mark_bit != 0 + } + + /// Returns `true` if the channel is empty. + pub(crate) fn is_empty(&self) -> bool { + let head = self.head.load(Ordering::SeqCst); + let tail = self.tail.load(Ordering::SeqCst); + + // Is the tail equal to the head? + // + // Note: If the head changes just before we load the tail, that means there was a moment + // when the channel was not empty, so it is safe to just return `false`. + (tail & !self.mark_bit) == head + } + + /// Returns `true` if the channel is full. + pub(crate) fn is_full(&self) -> bool { + let tail = self.tail.load(Ordering::SeqCst); + let head = self.head.load(Ordering::SeqCst); + + // Is the head lagging one lap behind tail? + // + // Note: If the tail changes just before we load the head, that means there was a moment + // when the channel was not full, so it is safe to just return `false`. + head.wrapping_add(self.one_lap) == tail & !self.mark_bit + } +} + +impl<T> Drop for Channel<T> { + fn drop(&mut self) { + // Get the index of the head. + let hix = self.head.load(Ordering::Relaxed) & (self.mark_bit - 1); + + // Loop over all slots that hold a message and drop them. + for i in 0..self.len() { + // Compute the index of the next slot holding a message. + let index = if hix + i < self.cap { hix + i } else { hix + i - self.cap }; + + unsafe { + debug_assert!(index < self.buffer.len()); + let slot = self.buffer.get_unchecked_mut(index); + let msg = &mut *slot.msg.get(); + msg.as_mut_ptr().drop_in_place(); + } + } + } +} diff --git a/library/std/src/sync/mpmc/context.rs b/library/std/src/sync/mpmc/context.rs new file mode 100644 index 00000000000..bbfc6ce00ff --- /dev/null +++ b/library/std/src/sync/mpmc/context.rs @@ -0,0 +1,155 @@ +//! Thread-local channel context. + +use super::select::Selected; +use super::waker::current_thread_id; + +use crate::cell::Cell; +use crate::ptr; +use crate::sync::atomic::{AtomicPtr, AtomicUsize, Ordering}; +use crate::sync::Arc; +use crate::thread::{self, Thread}; +use crate::time::Instant; + +/// Thread-local context. +#[derive(Debug, Clone)] +pub struct Context { + inner: Arc<Inner>, +} + +/// Inner representation of `Context`. +#[derive(Debug)] +struct Inner { + /// Selected operation. + select: AtomicUsize, + + /// A slot into which another thread may store a pointer to its `Packet`. + packet: AtomicPtr<()>, + + /// Thread handle. + thread: Thread, + + /// Thread id. + thread_id: usize, +} + +impl Context { + /// Creates a new context for the duration of the closure. + #[inline] + pub fn with<F, R>(f: F) -> R + where + F: FnOnce(&Context) -> R, + { + thread_local! { + /// Cached thread-local context. + static CONTEXT: Cell<Option<Context>> = Cell::new(Some(Context::new())); + } + + let mut f = Some(f); + let mut f = |cx: &Context| -> R { + let f = f.take().unwrap(); + f(cx) + }; + + CONTEXT + .try_with(|cell| match cell.take() { + None => f(&Context::new()), + Some(cx) => { + cx.reset(); + let res = f(&cx); + cell.set(Some(cx)); + res + } + }) + .unwrap_or_else(|_| f(&Context::new())) + } + + /// Creates a new `Context`. + #[cold] + fn new() -> Context { + Context { + inner: Arc::new(Inner { + select: AtomicUsize::new(Selected::Waiting.into()), + packet: AtomicPtr::new(ptr::null_mut()), + thread: thread::current(), + thread_id: current_thread_id(), + }), + } + } + + /// Resets `select` and `packet`. + #[inline] + fn reset(&self) { + self.inner.select.store(Selected::Waiting.into(), Ordering::Release); + self.inner.packet.store(ptr::null_mut(), Ordering::Release); + } + + /// Attempts to select an operation. + /// + /// On failure, the previously selected operation is returned. + #[inline] + pub fn try_select(&self, select: Selected) -> Result<(), Selected> { + self.inner + .select + .compare_exchange( + Selected::Waiting.into(), + select.into(), + Ordering::AcqRel, + Ordering::Acquire, + ) + .map(|_| ()) + .map_err(|e| e.into()) + } + + /// Stores a packet. + /// + /// This method must be called after `try_select` succeeds and there is a packet to provide. + #[inline] + pub fn store_packet(&self, packet: *mut ()) { + if !packet.is_null() { + self.inner.packet.store(packet, Ordering::Release); + } + } + + /// Waits until an operation is selected and returns it. + /// + /// If the deadline is reached, `Selected::Aborted` will be selected. + #[inline] + pub fn wait_until(&self, deadline: Option<Instant>) -> Selected { + loop { + // Check whether an operation has been selected. + let sel = Selected::from(self.inner.select.load(Ordering::Acquire)); + if sel != Selected::Waiting { + return sel; + } + + // If there's a deadline, park the current thread until the deadline is reached. + if let Some(end) = deadline { + let now = Instant::now(); + + if now < end { + thread::park_timeout(end - now); + } else { + // The deadline has been reached. Try aborting select. + return match self.try_select(Selected::Aborted) { + Ok(()) => Selected::Aborted, + Err(s) => s, + }; + } + } else { + thread::park(); + } + } + } + + /// Unparks the thread this context belongs to. + #[inline] + pub fn unpark(&self) { + self.inner.thread.unpark(); + } + + /// Returns the id of the thread this context belongs to. + #[inline] + pub fn thread_id(&self) -> usize { + self.inner.thread_id + } +} diff --git a/library/std/src/sync/mpmc/counter.rs b/library/std/src/sync/mpmc/counter.rs new file mode 100644 index 00000000000..a5a6bdc67f1 --- /dev/null +++ b/library/std/src/sync/mpmc/counter.rs @@ -0,0 +1,137 @@ +use crate::ops; +use crate::process; +use crate::sync::atomic::{AtomicBool, AtomicUsize, Ordering}; + +/// Reference counter internals. +struct Counter<C> { + /// The number of senders associated with the channel. + senders: AtomicUsize, + + /// The number of receivers associated with the channel. + receivers: AtomicUsize, + + /// Set to `true` if the last sender or the last receiver reference deallocates the channel. + destroy: AtomicBool, + + /// The internal channel. + chan: C, +} + +/// Wraps a channel into the reference counter. +pub(crate) fn new<C>(chan: C) -> (Sender<C>, Receiver<C>) { + let counter = Box::into_raw(Box::new(Counter { + senders: AtomicUsize::new(1), + receivers: AtomicUsize::new(1), + destroy: AtomicBool::new(false), + chan, + })); + let s = Sender { counter }; + let r = Receiver { counter }; + (s, r) +} + +/// The sending side. +pub(crate) struct Sender<C> { + counter: *mut Counter<C>, +} + +impl<C> Sender<C> { + /// Returns the internal `Counter`. + fn counter(&self) -> &Counter<C> { + unsafe { &*self.counter } + } + + /// Acquires another sender reference. + pub(crate) fn acquire(&self) -> Sender<C> { + let count = self.counter().senders.fetch_add(1, Ordering::Relaxed); + + // Cloning senders and calling `mem::forget` on the clones could potentially overflow the + // counter. It's very difficult to recover sensibly from such degenerate scenarios so we + // just abort when the count becomes very large. + if count > isize::MAX as usize { + process::abort(); + } + + Sender { counter: self.counter } + } + + /// Releases the sender reference. + /// + /// Function `disconnect` will be called if this is the last sender reference. + pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) { + if self.counter().senders.fetch_sub(1, Ordering::AcqRel) == 1 { + disconnect(&self.counter().chan); + + if self.counter().destroy.swap(true, Ordering::AcqRel) { + drop(Box::from_raw(self.counter)); + } + } + } +} + +impl<C> ops::Deref for Sender<C> { + type Target = C; + + fn deref(&self) -> &C { + &self.counter().chan + } +} + +impl<C> PartialEq for Sender<C> { + fn eq(&self, other: &Sender<C>) -> bool { + self.counter == other.counter + } +} + +/// The receiving side. +pub(crate) struct Receiver<C> { + counter: *mut Counter<C>, +} + +impl<C> Receiver<C> { + /// Returns the internal `Counter`. + fn counter(&self) -> &Counter<C> { + unsafe { &*self.counter } + } + + /// Acquires another receiver reference. + pub(crate) fn acquire(&self) -> Receiver<C> { + let count = self.counter().receivers.fetch_add(1, Ordering::Relaxed); + + // Cloning receivers and calling `mem::forget` on the clones could potentially overflow the + // counter. It's very difficult to recover sensibly from such degenerate scenarios so we + // just abort when the count becomes very large. + if count > isize::MAX as usize { + process::abort(); + } + + Receiver { counter: self.counter } + } + + /// Releases the receiver reference. + /// + /// Function `disconnect` will be called if this is the last receiver reference. + pub(crate) unsafe fn release<F: FnOnce(&C) -> bool>(&self, disconnect: F) { + if self.counter().receivers.fetch_sub(1, Ordering::AcqRel) == 1 { + disconnect(&self.counter().chan); + + if self.counter().destroy.swap(true, Ordering::AcqRel) { + drop(Box::from_raw(self.counter)); + } + } + } +} + +impl<C> ops::Deref for Receiver<C> { + type Target = C; + + fn deref(&self) -> &C { + &self.counter().chan + } +} + +impl<C> PartialEq for Receiver<C> { + fn eq(&self, other: &Receiver<C>) -> bool { + self.counter == other.counter + } +} diff --git a/library/std/src/sync/mpmc/error.rs b/library/std/src/sync/mpmc/error.rs new file mode 100644 index 00000000000..1b8a1f38797 --- /dev/null +++ b/library/std/src/sync/mpmc/error.rs @@ -0,0 +1,46 @@ +use crate::error; +use crate::fmt; + +pub use crate::sync::mpsc::{RecvError, RecvTimeoutError, SendError, TryRecvError, TrySendError}; + +/// An error returned from the [`send_timeout`] method. +/// +/// The error contains the message being sent so it can be recovered. +/// +/// [`send_timeout`]: super::Sender::send_timeout +#[derive(PartialEq, Eq, Clone, Copy)] +pub enum SendTimeoutError<T> { + /// The message could not be sent because the channel is full and the operation timed out. + /// + /// If this is a zero-capacity channel, then the error indicates that there was no receiver + /// available to receive the message and the operation timed out. + Timeout(T), + + /// The message could not be sent because the channel is disconnected. + Disconnected(T), +} + +impl<T> fmt::Debug for SendTimeoutError<T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + "SendTimeoutError(..)".fmt(f) + } +} + +impl<T> fmt::Display for SendTimeoutError<T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + match *self { + SendTimeoutError::Timeout(..) => "timed out waiting on send operation".fmt(f), + SendTimeoutError::Disconnected(..) => "sending on a disconnected channel".fmt(f), + } + } +} + +impl<T: Send> error::Error for SendTimeoutError<T> {} + +impl<T> From<SendError<T>> for SendTimeoutError<T> { + fn from(err: SendError<T>) -> SendTimeoutError<T> { + match err { + SendError(e) => SendTimeoutError::Disconnected(e), + } + } +} diff --git a/library/std/src/sync/mpmc/list.rs b/library/std/src/sync/mpmc/list.rs new file mode 100644 index 00000000000..2d5b2fb3b23 --- /dev/null +++ b/library/std/src/sync/mpmc/list.rs @@ -0,0 +1,638 @@ +//! Unbounded channel implemented as a linked list. + +use super::context::Context; +use super::error::*; +use super::select::{Operation, Selected, Token}; +use super::utils::{Backoff, CachePadded}; +use super::waker::SyncWaker; + +use crate::cell::UnsafeCell; +use crate::marker::PhantomData; +use crate::mem::MaybeUninit; +use crate::ptr; +use crate::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering}; +use crate::time::Instant; + +// Bits indicating the state of a slot: +// * If a message has been written into the slot, `WRITE` is set. +// * If a message has been read from the slot, `READ` is set. +// * If the block is being destroyed, `DESTROY` is set. +const WRITE: usize = 1; +const READ: usize = 2; +const DESTROY: usize = 4; + +// Each block covers one "lap" of indices. +const LAP: usize = 32; +// The maximum number of messages a block can hold. +const BLOCK_CAP: usize = LAP - 1; +// How many lower bits are reserved for metadata. +const SHIFT: usize = 1; +// Has two different purposes: +// * If set in head, indicates that the block is not the last one. +// * If set in tail, indicates that the channel is disconnected. +const MARK_BIT: usize = 1; + +/// A slot in a block. +struct Slot<T> { + /// The message. + msg: UnsafeCell<MaybeUninit<T>>, + + /// The state of the slot. + state: AtomicUsize, +} + +impl<T> Slot<T> { + /// Waits until a message is written into the slot. + fn wait_write(&self) { + let backoff = Backoff::new(); + while self.state.load(Ordering::Acquire) & WRITE == 0 { + backoff.snooze(); + } + } +} + +/// A block in a linked list. +/// +/// Each block in the list can hold up to `BLOCK_CAP` messages. +struct Block<T> { + /// The next block in the linked list. + next: AtomicPtr<Block<T>>, + + /// Slots for messages. + slots: [Slot<T>; BLOCK_CAP], +} + +impl<T> Block<T> { + /// Creates an empty block. + fn new() -> Block<T> { + // SAFETY: This is safe because: + // [1] `Block::next` (AtomicPtr) may be safely zero initialized. + // [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4]. + // [3] `Slot::msg` (UnsafeCell) may be safely zero initialized because it + // holds a MaybeUninit. + // [4] `Slot::state` (AtomicUsize) may be safely zero initialized. + unsafe { MaybeUninit::zeroed().assume_init() } + } + + /// Waits until the next pointer is set. + fn wait_next(&self) -> *mut Block<T> { + let backoff = Backoff::new(); + loop { + let next = self.next.load(Ordering::Acquire); + if !next.is_null() { + return next; + } + backoff.snooze(); + } + } + + /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block. + unsafe fn destroy(this: *mut Block<T>, start: usize) { + // It is not necessary to set the `DESTROY` bit in the last slot because that slot has + // begun destruction of the block. + for i in start..BLOCK_CAP - 1 { + let slot = (*this).slots.get_unchecked(i); + + // Mark the `DESTROY` bit if a thread is still using the slot. + if slot.state.load(Ordering::Acquire) & READ == 0 + && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0 + { + // If a thread is still using the slot, it will continue destruction of the block. + return; + } + } + + // No thread is using the block, now it is safe to destroy it. + drop(Box::from_raw(this)); + } +} + +/// A position in a channel. +#[derive(Debug)] +struct Position<T> { + /// The index in the channel. + index: AtomicUsize, + + /// The block in the linked list. + block: AtomicPtr<Block<T>>, +} + +/// The token type for the list flavor. +#[derive(Debug)] +pub(crate) struct ListToken { + /// The block of slots. + block: *const u8, + + /// The offset into the block. + offset: usize, +} + +impl Default for ListToken { + #[inline] + fn default() -> Self { + ListToken { block: ptr::null(), offset: 0 } + } +} + +/// Unbounded channel implemented as a linked list. +/// +/// Each message sent into the channel is assigned a sequence number, i.e. an index. Indices are +/// represented as numbers of type `usize` and wrap on overflow. +/// +/// Consecutive messages are grouped into blocks in order to put less pressure on the allocator and +/// improve cache efficiency. +pub(crate) struct Channel<T> { + /// The head of the channel. + head: CachePadded<Position<T>>, + + /// The tail of the channel. + tail: CachePadded<Position<T>>, + + /// Receivers waiting while the channel is empty and not disconnected. + receivers: SyncWaker, + + /// Indicates that dropping a `Channel<T>` may drop messages of type `T`. + _marker: PhantomData<T>, +} + +impl<T> Channel<T> { + /// Creates a new unbounded channel. + pub(crate) fn new() -> Self { + Channel { + head: CachePadded::new(Position { + block: AtomicPtr::new(ptr::null_mut()), + index: AtomicUsize::new(0), + }), + tail: CachePadded::new(Position { + block: AtomicPtr::new(ptr::null_mut()), + index: AtomicUsize::new(0), + }), + receivers: SyncWaker::new(), + _marker: PhantomData, + } + } + + /// Attempts to reserve a slot for sending a message. + fn start_send(&self, token: &mut Token) -> bool { + let backoff = Backoff::new(); + let mut tail = self.tail.index.load(Ordering::Acquire); + let mut block = self.tail.block.load(Ordering::Acquire); + let mut next_block = None; + + loop { + // Check if the channel is disconnected. + if tail & MARK_BIT != 0 { + token.list.block = ptr::null(); + return true; + } + + // Calculate the offset of the index into the block. + let offset = (tail >> SHIFT) % LAP; + + // If we reached the end of the block, wait until the next one is installed. + if offset == BLOCK_CAP { + backoff.snooze(); + tail = self.tail.index.load(Ordering::Acquire); + block = self.tail.block.load(Ordering::Acquire); + continue; + } + + // If we're going to have to install the next block, allocate it in advance in order to + // make the wait for other threads as short as possible. + if offset + 1 == BLOCK_CAP && next_block.is_none() { + next_block = Some(Box::new(Block::<T>::new())); + } + + // If this is the first message to be sent into the channel, we need to allocate the + // first block and install it. + if block.is_null() { + let new = Box::into_raw(Box::new(Block::<T>::new())); + + if self + .tail + .block + .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed) + .is_ok() + { + self.head.block.store(new, Ordering::Release); + block = new; + } else { + next_block = unsafe { Some(Box::from_raw(new)) }; + tail = self.tail.index.load(Ordering::Acquire); + block = self.tail.block.load(Ordering::Acquire); + continue; + } + } + + let new_tail = tail + (1 << SHIFT); + + // Try advancing the tail forward. + match self.tail.index.compare_exchange_weak( + tail, + new_tail, + Ordering::SeqCst, + Ordering::Acquire, + ) { + Ok(_) => unsafe { + // If we've reached the end of the block, install the next one. + if offset + 1 == BLOCK_CAP { + let next_block = Box::into_raw(next_block.unwrap()); + self.tail.block.store(next_block, Ordering::Release); + self.tail.index.fetch_add(1 << SHIFT, Ordering::Release); + (*block).next.store(next_block, Ordering::Release); + } + + token.list.block = block as *const u8; + token.list.offset = offset; + return true; + }, + Err(_) => { + backoff.spin(); + tail = self.tail.index.load(Ordering::Acquire); + block = self.tail.block.load(Ordering::Acquire); + } + } + } + } + + /// Writes a message into the channel. + pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> { + // If there is no slot, the channel is disconnected. + if token.list.block.is_null() { + return Err(msg); + } + + // Write the message into the slot. + let block = token.list.block as *mut Block<T>; + let offset = token.list.offset; + let slot = (*block).slots.get_unchecked(offset); + slot.msg.get().write(MaybeUninit::new(msg)); + slot.state.fetch_or(WRITE, Ordering::Release); + + // Wake a sleeping receiver. + self.receivers.notify(); + Ok(()) + } + + /// Attempts to reserve a slot for receiving a message. + fn start_recv(&self, token: &mut Token) -> bool { + let backoff = Backoff::new(); + let mut head = self.head.index.load(Ordering::Acquire); + let mut block = self.head.block.load(Ordering::Acquire); + + loop { + // Calculate the offset of the index into the block. + let offset = (head >> SHIFT) % LAP; + + // If we reached the end of the block, wait until the next one is installed. + if offset == BLOCK_CAP { + backoff.snooze(); + head = self.head.index.load(Ordering::Acquire); + block = self.head.block.load(Ordering::Acquire); + continue; + } + + let mut new_head = head + (1 << SHIFT); + + if new_head & MARK_BIT == 0 { + atomic::fence(Ordering::SeqCst); + let tail = self.tail.index.load(Ordering::Relaxed); + + // If the tail equals the head, that means the channel is empty. + if head >> SHIFT == tail >> SHIFT { + // If the channel is disconnected... + if tail & MARK_BIT != 0 { + // ...then receive an error. + token.list.block = ptr::null(); + return true; + } else { + // Otherwise, the receive operation is not ready. + return false; + } + } + + // If head and tail are not in the same block, set `MARK_BIT` in head. + if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP { + new_head |= MARK_BIT; + } + } + + // The block can be null here only if the first message is being sent into the channel. + // In that case, just wait until it gets initialized. + if block.is_null() { + backoff.snooze(); + head = self.head.index.load(Ordering::Acquire); + block = self.head.block.load(Ordering::Acquire); + continue; + } + + // Try moving the head index forward. + match self.head.index.compare_exchange_weak( + head, + new_head, + Ordering::SeqCst, + Ordering::Acquire, + ) { + Ok(_) => unsafe { + // If we've reached the end of the block, move to the next one. + if offset + 1 == BLOCK_CAP { + let next = (*block).wait_next(); + let mut next_index = (new_head & !MARK_BIT).wrapping_add(1 << SHIFT); + if !(*next).next.load(Ordering::Relaxed).is_null() { + next_index |= MARK_BIT; + } + + self.head.block.store(next, Ordering::Release); + self.head.index.store(next_index, Ordering::Release); + } + + token.list.block = block as *const u8; + token.list.offset = offset; + return true; + }, + Err(_) => { + backoff.spin(); + head = self.head.index.load(Ordering::Acquire); + block = self.head.block.load(Ordering::Acquire); + } + } + } + } + + /// Reads a message from the channel. + pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> { + if token.list.block.is_null() { + // The channel is disconnected. + return Err(()); + } + + // Read the message. + let block = token.list.block as *mut Block<T>; + let offset = token.list.offset; + let slot = (*block).slots.get_unchecked(offset); + slot.wait_write(); + let msg = slot.msg.get().read().assume_init(); + + // Destroy the block if we've reached the end, or if another thread wanted to destroy but + // couldn't because we were busy reading from the slot. + if offset + 1 == BLOCK_CAP { + Block::destroy(block, 0); + } else if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 { + Block::destroy(block, offset + 1); + } + + Ok(msg) + } + + /// Attempts to send a message into the channel. + pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> { + self.send(msg, None).map_err(|err| match err { + SendTimeoutError::Disconnected(msg) => TrySendError::Disconnected(msg), + SendTimeoutError::Timeout(_) => unreachable!(), + }) + } + + /// Sends a message into the channel. + pub(crate) fn send( + &self, + msg: T, + _deadline: Option<Instant>, + ) -> Result<(), SendTimeoutError<T>> { + let token = &mut Token::default(); + assert!(self.start_send(token)); + unsafe { self.write(token, msg).map_err(SendTimeoutError::Disconnected) } + } + + /// Attempts to receive a message without blocking. + pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> { + let token = &mut Token::default(); + + if self.start_recv(token) { + unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) } + } else { + Err(TryRecvError::Empty) + } + } + + /// Receives a message from the channel. + pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> { + let token = &mut Token::default(); + loop { + if self.start_recv(token) { + unsafe { + return self.read(token).map_err(|_| RecvTimeoutError::Disconnected); + } + } + + if let Some(d) = deadline { + if Instant::now() >= d { + return Err(RecvTimeoutError::Timeout); + } + } + + // Prepare for blocking until a sender wakes us up. + Context::with(|cx| { + let oper = Operation::hook(token); + self.receivers.register(oper, cx); + + // Has the channel become ready just now? + if !self.is_empty() || self.is_disconnected() { + let _ = cx.try_select(Selected::Aborted); + } + + // Block the current thread. + let sel = cx.wait_until(deadline); + + match sel { + Selected::Waiting => unreachable!(), + Selected::Aborted | Selected::Disconnected => { + self.receivers.unregister(oper).unwrap(); + // If the channel was disconnected, we still have to check for remaining + // messages. + } + Selected::Operation(_) => {} + } + }); + } + } + + /// Returns the current number of messages inside the channel. + pub(crate) fn len(&self) -> usize { + loop { + // Load the tail index, then load the head index. + let mut tail = self.tail.index.load(Ordering::SeqCst); + let mut head = self.head.index.load(Ordering::SeqCst); + + // If the tail index didn't change, we've got consistent indices to work with. + if self.tail.index.load(Ordering::SeqCst) == tail { + // Erase the lower bits. + tail &= !((1 << SHIFT) - 1); + head &= !((1 << SHIFT) - 1); + + // Fix up indices if they fall onto block ends. + if (tail >> SHIFT) & (LAP - 1) == LAP - 1 { + tail = tail.wrapping_add(1 << SHIFT); + } + if (head >> SHIFT) & (LAP - 1) == LAP - 1 { + head = head.wrapping_add(1 << SHIFT); + } + + // Rotate indices so that head falls into the first block. + let lap = (head >> SHIFT) / LAP; + tail = tail.wrapping_sub((lap * LAP) << SHIFT); + head = head.wrapping_sub((lap * LAP) << SHIFT); + + // Remove the lower bits. + tail >>= SHIFT; + head >>= SHIFT; + + // Return the difference minus the number of blocks between tail and head. + return tail - head - tail / LAP; + } + } + } + + /// Returns the capacity of the channel. + pub(crate) fn capacity(&self) -> Option<usize> { + None + } + + /// Disconnects senders and wakes up all blocked receivers. + /// + /// Returns `true` if this call disconnected the channel. + pub(crate) fn disconnect_senders(&self) -> bool { + let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst); + + if tail & MARK_BIT == 0 { + self.receivers.disconnect(); + true + } else { + false + } + } + + /// Disconnects receivers. + /// + /// Returns `true` if this call disconnected the channel. + pub(crate) fn disconnect_receivers(&self) -> bool { + let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst); + + if tail & MARK_BIT == 0 { + // If receivers are dropped first, discard all messages to free + // memory eagerly. + self.discard_all_messages(); + true + } else { + false + } + } + + /// Discards all messages. + /// + /// This method should only be called when all receivers are dropped. + fn discard_all_messages(&self) { + let backoff = Backoff::new(); + let mut tail = self.tail.index.load(Ordering::Acquire); + loop { + let offset = (tail >> SHIFT) % LAP; + if offset != BLOCK_CAP { + break; + } + + // New updates to tail will be rejected by MARK_BIT and aborted unless it's + // at boundary. We need to wait for the updates take affect otherwise there + // can be memory leaks. + backoff.snooze(); + tail = self.tail.index.load(Ordering::Acquire); + } + + let mut head = self.head.index.load(Ordering::Acquire); + let mut block = self.head.block.load(Ordering::Acquire); + + unsafe { + // Drop all messages between head and tail and deallocate the heap-allocated blocks. + while head >> SHIFT != tail >> SHIFT { + let offset = (head >> SHIFT) % LAP; + + if offset < BLOCK_CAP { + // Drop the message in the slot. + let slot = (*block).slots.get_unchecked(offset); + slot.wait_write(); + let p = &mut *slot.msg.get(); + p.as_mut_ptr().drop_in_place(); + } else { + (*block).wait_next(); + // Deallocate the block and move to the next one. + let next = (*block).next.load(Ordering::Acquire); + drop(Box::from_raw(block)); + block = next; + } + + head = head.wrapping_add(1 << SHIFT); + } + + // Deallocate the last remaining block. + if !block.is_null() { + drop(Box::from_raw(block)); + } + } + head &= !MARK_BIT; + self.head.block.store(ptr::null_mut(), Ordering::Release); + self.head.index.store(head, Ordering::Release); + } + + /// Returns `true` if the channel is disconnected. + pub(crate) fn is_disconnected(&self) -> bool { + self.tail.index.load(Ordering::SeqCst) & MARK_BIT != 0 + } + + /// Returns `true` if the channel is empty. + pub(crate) fn is_empty(&self) -> bool { + let head = self.head.index.load(Ordering::SeqCst); + let tail = self.tail.index.load(Ordering::SeqCst); + head >> SHIFT == tail >> SHIFT + } + + /// Returns `true` if the channel is full. + pub(crate) fn is_full(&self) -> bool { + false + } +} + +impl<T> Drop for Channel<T> { + fn drop(&mut self) { + let mut head = self.head.index.load(Ordering::Relaxed); + let mut tail = self.tail.index.load(Ordering::Relaxed); + let mut block = self.head.block.load(Ordering::Relaxed); + + // Erase the lower bits. + head &= !((1 << SHIFT) - 1); + tail &= !((1 << SHIFT) - 1); + + unsafe { + // Drop all messages between head and tail and deallocate the heap-allocated blocks. + while head != tail { + let offset = (head >> SHIFT) % LAP; + + if offset < BLOCK_CAP { + // Drop the message in the slot. + let slot = (*block).slots.get_unchecked(offset); + let p = &mut *slot.msg.get(); + p.as_mut_ptr().drop_in_place(); + } else { + // Deallocate the block and move to the next one. + let next = (*block).next.load(Ordering::Relaxed); + drop(Box::from_raw(block)); + block = next; + } + + head = head.wrapping_add(1 << SHIFT); + } + + // Deallocate the last remaining block. + if !block.is_null() { + drop(Box::from_raw(block)); + } + } + } +} diff --git a/library/std/src/sync/mpmc/mod.rs b/library/std/src/sync/mpmc/mod.rs new file mode 100644 index 00000000000..cef99c58843 --- /dev/null +++ b/library/std/src/sync/mpmc/mod.rs @@ -0,0 +1,430 @@ +//! Multi-producer multi-consumer channels. + +// This module is not currently exposed publicly, but is used +// as the implementation for the channels in `sync::mpsc`. The +// implementation comes from the crossbeam-channel crate: +// +// Copyright (c) 2019 The Crossbeam Project Developers +// +// Permission is hereby granted, free of charge, to any +// person obtaining a copy of this software and associated +// documentation files (the "Software"), to deal in the +// Software without restriction, including without +// limitation the rights to use, copy, modify, merge, +// publish, distribute, sublicense, and/or sell copies of +// the Software, and to permit persons to whom the Software +// is furnished to do so, subject to the following +// conditions: +// +// The above copyright notice and this permission notice +// shall be included in all copies or substantial portions +// of the Software. +// +// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF +// ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED +// TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A +// PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT +// SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY +// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION +// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR +// IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER +// DEALINGS IN THE SOFTWARE. + +mod array; +mod context; +mod counter; +mod error; +mod list; +mod select; +mod utils; +mod waker; +mod zero; + +use crate::fmt; +use crate::panic::{RefUnwindSafe, UnwindSafe}; +use crate::time::{Duration, Instant}; +use error::*; + +/// Creates a channel of unbounded capacity. +/// +/// This channel has a growable buffer that can hold any number of messages at a time. +pub fn channel<T>() -> (Sender<T>, Receiver<T>) { + let (s, r) = counter::new(list::Channel::new()); + let s = Sender { flavor: SenderFlavor::List(s) }; + let r = Receiver { flavor: ReceiverFlavor::List(r) }; + (s, r) +} + +/// Creates a channel of bounded capacity. +/// +/// This channel has a buffer that can hold at most `cap` messages at a time. +/// +/// A special case is zero-capacity channel, which cannot hold any messages. Instead, send and +/// receive operations must appear at the same time in order to pair up and pass the message over. +pub fn sync_channel<T>(cap: usize) -> (Sender<T>, Receiver<T>) { + if cap == 0 { + let (s, r) = counter::new(zero::Channel::new()); + let s = Sender { flavor: SenderFlavor::Zero(s) }; + let r = Receiver { flavor: ReceiverFlavor::Zero(r) }; + (s, r) + } else { + let (s, r) = counter::new(array::Channel::with_capacity(cap)); + let s = Sender { flavor: SenderFlavor::Array(s) }; + let r = Receiver { flavor: ReceiverFlavor::Array(r) }; + (s, r) + } +} + +/// The sending side of a channel. +pub struct Sender<T> { + flavor: SenderFlavor<T>, +} + +/// Sender flavors. +enum SenderFlavor<T> { + /// Bounded channel based on a preallocated array. + Array(counter::Sender<array::Channel<T>>), + + /// Unbounded channel implemented as a linked list. + List(counter::Sender<list::Channel<T>>), + + /// Zero-capacity channel. + Zero(counter::Sender<zero::Channel<T>>), +} + +unsafe impl<T: Send> Send for Sender<T> {} +unsafe impl<T: Send> Sync for Sender<T> {} + +impl<T> UnwindSafe for Sender<T> {} +impl<T> RefUnwindSafe for Sender<T> {} + +impl<T> Sender<T> { + /// Attempts to send a message into the channel without blocking. + /// + /// This method will either send a message into the channel immediately or return an error if + /// the channel is full or disconnected. The returned error contains the original message. + /// + /// If called on a zero-capacity channel, this method will send the message only if there + /// happens to be a receive operation on the other side of the channel at the same time. + pub fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> { + match &self.flavor { + SenderFlavor::Array(chan) => chan.try_send(msg), + SenderFlavor::List(chan) => chan.try_send(msg), + SenderFlavor::Zero(chan) => chan.try_send(msg), + } + } + + /// Blocks the current thread until a message is sent or the channel is disconnected. + /// + /// If the channel is full and not disconnected, this call will block until the send operation + /// can proceed. If the channel becomes disconnected, this call will wake up and return an + /// error. The returned error contains the original message. + /// + /// If called on a zero-capacity channel, this method will wait for a receive operation to + /// appear on the other side of the channel. + pub fn send(&self, msg: T) -> Result<(), SendError<T>> { + match &self.flavor { + SenderFlavor::Array(chan) => chan.send(msg, None), + SenderFlavor::List(chan) => chan.send(msg, None), + SenderFlavor::Zero(chan) => chan.send(msg, None), + } + .map_err(|err| match err { + SendTimeoutError::Disconnected(msg) => SendError(msg), + SendTimeoutError::Timeout(_) => unreachable!(), + }) + } +} + +// The methods below are not used by `sync::mpsc`, but +// are useful and we'll likely want to expose them +// eventually +#[allow(unused)] +impl<T> Sender<T> { + /// Waits for a message to be sent into the channel, but only for a limited time. + /// + /// If the channel is full and not disconnected, this call will block until the send operation + /// can proceed or the operation times out. If the channel becomes disconnected, this call will + /// wake up and return an error. The returned error contains the original message. + /// + /// If called on a zero-capacity channel, this method will wait for a receive operation to + /// appear on the other side of the channel. + pub fn send_timeout(&self, msg: T, timeout: Duration) -> Result<(), SendTimeoutError<T>> { + match Instant::now().checked_add(timeout) { + Some(deadline) => self.send_deadline(msg, deadline), + // So far in the future that it's practically the same as waiting indefinitely. + None => self.send(msg).map_err(SendTimeoutError::from), + } + } + + /// Waits for a message to be sent into the channel, but only until a given deadline. + /// + /// If the channel is full and not disconnected, this call will block until the send operation + /// can proceed or the operation times out. If the channel becomes disconnected, this call will + /// wake up and return an error. The returned error contains the original message. + /// + /// If called on a zero-capacity channel, this method will wait for a receive operation to + /// appear on the other side of the channel. + pub fn send_deadline(&self, msg: T, deadline: Instant) -> Result<(), SendTimeoutError<T>> { + match &self.flavor { + SenderFlavor::Array(chan) => chan.send(msg, Some(deadline)), + SenderFlavor::List(chan) => chan.send(msg, Some(deadline)), + SenderFlavor::Zero(chan) => chan.send(msg, Some(deadline)), + } + } + + /// Returns `true` if the channel is empty. + /// + /// Note: Zero-capacity channels are always empty. + pub fn is_empty(&self) -> bool { + match &self.flavor { + SenderFlavor::Array(chan) => chan.is_empty(), + SenderFlavor::List(chan) => chan.is_empty(), + SenderFlavor::Zero(chan) => chan.is_empty(), + } + } + + /// Returns `true` if the channel is full. + /// + /// Note: Zero-capacity channels are always full. + pub fn is_full(&self) -> bool { + match &self.flavor { + SenderFlavor::Array(chan) => chan.is_full(), + SenderFlavor::List(chan) => chan.is_full(), + SenderFlavor::Zero(chan) => chan.is_full(), + } + } + + /// Returns the number of messages in the channel. + pub fn len(&self) -> usize { + match &self.flavor { + SenderFlavor::Array(chan) => chan.len(), + SenderFlavor::List(chan) => chan.len(), + SenderFlavor::Zero(chan) => chan.len(), + } + } + + /// If the channel is bounded, returns its capacity. + pub fn capacity(&self) -> Option<usize> { + match &self.flavor { + SenderFlavor::Array(chan) => chan.capacity(), + SenderFlavor::List(chan) => chan.capacity(), + SenderFlavor::Zero(chan) => chan.capacity(), + } + } + + /// Returns `true` if senders belong to the same channel. + pub fn same_channel(&self, other: &Sender<T>) -> bool { + match (&self.flavor, &other.flavor) { + (SenderFlavor::Array(ref a), SenderFlavor::Array(ref b)) => a == b, + (SenderFlavor::List(ref a), SenderFlavor::List(ref b)) => a == b, + (SenderFlavor::Zero(ref a), SenderFlavor::Zero(ref b)) => a == b, + _ => false, + } + } +} + +impl<T> Drop for Sender<T> { + fn drop(&mut self) { + unsafe { + match &self.flavor { + SenderFlavor::Array(chan) => chan.release(|c| c.disconnect()), + SenderFlavor::List(chan) => chan.release(|c| c.disconnect_senders()), + SenderFlavor::Zero(chan) => chan.release(|c| c.disconnect()), + } + } + } +} + +impl<T> Clone for Sender<T> { + fn clone(&self) -> Self { + let flavor = match &self.flavor { + SenderFlavor::Array(chan) => SenderFlavor::Array(chan.acquire()), + SenderFlavor::List(chan) => SenderFlavor::List(chan.acquire()), + SenderFlavor::Zero(chan) => SenderFlavor::Zero(chan.acquire()), + }; + + Sender { flavor } + } +} + +impl<T> fmt::Debug for Sender<T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.pad("Sender { .. }") + } +} + +/// The receiving side of a channel. +pub struct Receiver<T> { + flavor: ReceiverFlavor<T>, +} + +/// Receiver flavors. +enum ReceiverFlavor<T> { + /// Bounded channel based on a preallocated array. + Array(counter::Receiver<array::Channel<T>>), + + /// Unbounded channel implemented as a linked list. + List(counter::Receiver<list::Channel<T>>), + + /// Zero-capacity channel. + Zero(counter::Receiver<zero::Channel<T>>), +} + +unsafe impl<T: Send> Send for Receiver<T> {} +unsafe impl<T: Send> Sync for Receiver<T> {} + +impl<T> UnwindSafe for Receiver<T> {} +impl<T> RefUnwindSafe for Receiver<T> {} + +impl<T> Receiver<T> { + /// Attempts to receive a message from the channel without blocking. + /// + /// This method will either receive a message from the channel immediately or return an error + /// if the channel is empty. + /// + /// If called on a zero-capacity channel, this method will receive a message only if there + /// happens to be a send operation on the other side of the channel at the same time. + pub fn try_recv(&self) -> Result<T, TryRecvError> { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.try_recv(), + ReceiverFlavor::List(chan) => chan.try_recv(), + ReceiverFlavor::Zero(chan) => chan.try_recv(), + } + } + + /// Blocks the current thread until a message is received or the channel is empty and + /// disconnected. + /// + /// If the channel is empty and not disconnected, this call will block until the receive + /// operation can proceed. If the channel is empty and becomes disconnected, this call will + /// wake up and return an error. + /// + /// If called on a zero-capacity channel, this method will wait for a send operation to appear + /// on the other side of the channel. + pub fn recv(&self) -> Result<T, RecvError> { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.recv(None), + ReceiverFlavor::List(chan) => chan.recv(None), + ReceiverFlavor::Zero(chan) => chan.recv(None), + } + .map_err(|_| RecvError) + } + + /// Waits for a message to be received from the channel, but only for a limited time. + /// + /// If the channel is empty and not disconnected, this call will block until the receive + /// operation can proceed or the operation times out. If the channel is empty and becomes + /// disconnected, this call will wake up and return an error. + /// + /// If called on a zero-capacity channel, this method will wait for a send operation to appear + /// on the other side of the channel. + pub fn recv_timeout(&self, timeout: Duration) -> Result<T, RecvTimeoutError> { + match Instant::now().checked_add(timeout) { + Some(deadline) => self.recv_deadline(deadline), + // So far in the future that it's practically the same as waiting indefinitely. + None => self.recv().map_err(RecvTimeoutError::from), + } + } + + /// Waits for a message to be received from the channel, but only for a limited time. + /// + /// If the channel is empty and not disconnected, this call will block until the receive + /// operation can proceed or the operation times out. If the channel is empty and becomes + /// disconnected, this call will wake up and return an error. + /// + /// If called on a zero-capacity channel, this method will wait for a send operation to appear + /// on the other side of the channel. + pub fn recv_deadline(&self, deadline: Instant) -> Result<T, RecvTimeoutError> { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.recv(Some(deadline)), + ReceiverFlavor::List(chan) => chan.recv(Some(deadline)), + ReceiverFlavor::Zero(chan) => chan.recv(Some(deadline)), + } + } +} + +// The methods below are not used by `sync::mpsc`, but +// are useful and we'll likely want to expose them +// eventually +#[allow(unused)] +impl<T> Receiver<T> { + /// Returns `true` if the channel is empty. + /// + /// Note: Zero-capacity channels are always empty. + pub fn is_empty(&self) -> bool { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.is_empty(), + ReceiverFlavor::List(chan) => chan.is_empty(), + ReceiverFlavor::Zero(chan) => chan.is_empty(), + } + } + + /// Returns `true` if the channel is full. + /// + /// Note: Zero-capacity channels are always full. + pub fn is_full(&self) -> bool { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.is_full(), + ReceiverFlavor::List(chan) => chan.is_full(), + ReceiverFlavor::Zero(chan) => chan.is_full(), + } + } + + /// Returns the number of messages in the channel. + pub fn len(&self) -> usize { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.len(), + ReceiverFlavor::List(chan) => chan.len(), + ReceiverFlavor::Zero(chan) => chan.len(), + } + } + + /// If the channel is bounded, returns its capacity. + pub fn capacity(&self) -> Option<usize> { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.capacity(), + ReceiverFlavor::List(chan) => chan.capacity(), + ReceiverFlavor::Zero(chan) => chan.capacity(), + } + } + + /// Returns `true` if receivers belong to the same channel. + pub fn same_channel(&self, other: &Receiver<T>) -> bool { + match (&self.flavor, &other.flavor) { + (ReceiverFlavor::Array(a), ReceiverFlavor::Array(b)) => a == b, + (ReceiverFlavor::List(a), ReceiverFlavor::List(b)) => a == b, + (ReceiverFlavor::Zero(a), ReceiverFlavor::Zero(b)) => a == b, + _ => false, + } + } +} + +impl<T> Drop for Receiver<T> { + fn drop(&mut self) { + unsafe { + match &self.flavor { + ReceiverFlavor::Array(chan) => chan.release(|c| c.disconnect()), + ReceiverFlavor::List(chan) => chan.release(|c| c.disconnect_receivers()), + ReceiverFlavor::Zero(chan) => chan.release(|c| c.disconnect()), + } + } + } +} + +impl<T> Clone for Receiver<T> { + fn clone(&self) -> Self { + let flavor = match &self.flavor { + ReceiverFlavor::Array(chan) => ReceiverFlavor::Array(chan.acquire()), + ReceiverFlavor::List(chan) => ReceiverFlavor::List(chan.acquire()), + ReceiverFlavor::Zero(chan) => ReceiverFlavor::Zero(chan.acquire()), + }; + + Receiver { flavor } + } +} + +impl<T> fmt::Debug for Receiver<T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.pad("Receiver { .. }") + } +} diff --git a/library/std/src/sync/mpmc/select.rs b/library/std/src/sync/mpmc/select.rs new file mode 100644 index 00000000000..56a83fee2e1 --- /dev/null +++ b/library/std/src/sync/mpmc/select.rs @@ -0,0 +1,71 @@ +/// Temporary data that gets initialized during a blocking operation, and is consumed by +/// `read` or `write`. +/// +/// Each field contains data associated with a specific channel flavor. +#[derive(Debug, Default)] +pub struct Token { + pub(crate) array: super::array::ArrayToken, + pub(crate) list: super::list::ListToken, + #[allow(dead_code)] + pub(crate) zero: super::zero::ZeroToken, +} + +/// Identifier associated with an operation by a specific thread on a specific channel. +#[derive(Debug, Clone, Copy, PartialEq, Eq)] +pub struct Operation(usize); + +impl Operation { + /// Creates an operation identifier from a mutable reference. + /// + /// This function essentially just turns the address of the reference into a number. The + /// reference should point to a variable that is specific to the thread and the operation, + /// and is alive for the entire duration of a blocking operation. + #[inline] + pub fn hook<T>(r: &mut T) -> Operation { + let val = r as *mut T as usize; + // Make sure that the pointer address doesn't equal the numerical representation of + // `Selected::{Waiting, Aborted, Disconnected}`. + assert!(val > 2); + Operation(val) + } +} + +/// Current state of a blocking operation. +#[derive(Debug, Clone, Copy, PartialEq, Eq)] +pub enum Selected { + /// Still waiting for an operation. + Waiting, + + /// The attempt to block the current thread has been aborted. + Aborted, + + /// An operation became ready because a channel is disconnected. + Disconnected, + + /// An operation became ready because a message can be sent or received. + Operation(Operation), +} + +impl From<usize> for Selected { + #[inline] + fn from(val: usize) -> Selected { + match val { + 0 => Selected::Waiting, + 1 => Selected::Aborted, + 2 => Selected::Disconnected, + oper => Selected::Operation(Operation(oper)), + } + } +} + +impl Into<usize> for Selected { + #[inline] + fn into(self) -> usize { + match self { + Selected::Waiting => 0, + Selected::Aborted => 1, + Selected::Disconnected => 2, + Selected::Operation(Operation(val)) => val, + } + } +} diff --git a/library/std/src/sync/mpmc/utils.rs b/library/std/src/sync/mpmc/utils.rs new file mode 100644 index 00000000000..d0904b4b94c --- /dev/null +++ b/library/std/src/sync/mpmc/utils.rs @@ -0,0 +1,144 @@ +use crate::cell::Cell; +use crate::ops::{Deref, DerefMut}; + +/// Pads and aligns a value to the length of a cache line. +#[derive(Clone, Copy, Default, Hash, PartialEq, Eq)] +// Starting from Intel's Sandy Bridge, spatial prefetcher is now pulling pairs of 64-byte cache +// lines at a time, so we have to align to 128 bytes rather than 64. +// +// Sources: +// - https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-optimization-manual.pdf +// - https://github.com/facebook/folly/blob/1b5288e6eea6df074758f877c849b6e73bbb9fbb/folly/lang/Align.h#L107 +// +// ARM's big.LITTLE architecture has asymmetric cores and "big" cores have 128-byte cache line size. +// +// Sources: +// - https://www.mono-project.com/news/2016/09/12/arm64-icache/ +// +// powerpc64 has 128-byte cache line size. +// +// Sources: +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_ppc64x.go#L9 +#[cfg_attr( + any(target_arch = "x86_64", target_arch = "aarch64", target_arch = "powerpc64",), + repr(align(128)) +)] +// arm, mips, mips64, and riscv64 have 32-byte cache line size. +// +// Sources: +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_arm.go#L7 +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips.go#L7 +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mipsle.go#L7 +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips64x.go#L9 +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_riscv64.go#L7 +#[cfg_attr( + any( + target_arch = "arm", + target_arch = "mips", + target_arch = "mips64", + target_arch = "riscv64", + ), + repr(align(32)) +)] +// s390x has 256-byte cache line size. +// +// Sources: +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_s390x.go#L7 +#[cfg_attr(target_arch = "s390x", repr(align(256)))] +// x86 and wasm have 64-byte cache line size. +// +// Sources: +// - https://github.com/golang/go/blob/dda2991c2ea0c5914714469c4defc2562a907230/src/internal/cpu/cpu_x86.go#L9 +// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_wasm.go#L7 +// +// All others are assumed to have 64-byte cache line size. +#[cfg_attr( + not(any( + target_arch = "x86_64", + target_arch = "aarch64", + target_arch = "powerpc64", + target_arch = "arm", + target_arch = "mips", + target_arch = "mips64", + target_arch = "riscv64", + target_arch = "s390x", + )), + repr(align(64)) +)] +pub struct CachePadded<T> { + value: T, +} + +impl<T> CachePadded<T> { + /// Pads and aligns a value to the length of a cache line. + pub fn new(value: T) -> CachePadded<T> { + CachePadded::<T> { value } + } +} + +impl<T> Deref for CachePadded<T> { + type Target = T; + + fn deref(&self) -> &T { + &self.value + } +} + +impl<T> DerefMut for CachePadded<T> { + fn deref_mut(&mut self) -> &mut T { + &mut self.value + } +} + +const SPIN_LIMIT: u32 = 6; +const YIELD_LIMIT: u32 = 10; + +/// Performs exponential backoff in spin loops. +pub struct Backoff { + step: Cell<u32>, +} + +impl Backoff { + /// Creates a new `Backoff`. + pub fn new() -> Self { + Backoff { step: Cell::new(0) } + } + + /// Backs off in a lock-free loop. + /// + /// This method should be used when we need to retry an operation because another thread made + /// progress. + #[inline] + pub fn spin(&self) { + let step = self.step.get().min(SPIN_LIMIT); + for _ in 0..step.pow(2) { + crate::hint::spin_loop(); + } + + if self.step.get() <= SPIN_LIMIT { + self.step.set(self.step.get() + 1); + } + } + + /// Backs off in a blocking loop. + #[inline] + pub fn snooze(&self) { + if self.step.get() <= SPIN_LIMIT { + for _ in 0..self.step.get().pow(2) { + crate::hint::spin_loop() + } + } else { + crate::thread::yield_now(); + } + + if self.step.get() <= YIELD_LIMIT { + self.step.set(self.step.get() + 1); + } + } + + /// Returns `true` if exponential backoff has completed and blocking the thread is advised. + #[inline] + pub fn is_completed(&self) -> bool { + self.step.get() > YIELD_LIMIT + } +} diff --git a/library/std/src/sync/mpmc/waker.rs b/library/std/src/sync/mpmc/waker.rs new file mode 100644 index 00000000000..4912ca4f815 --- /dev/null +++ b/library/std/src/sync/mpmc/waker.rs @@ -0,0 +1,204 @@ +//! Waking mechanism for threads blocked on channel operations. + +use super::context::Context; +use super::select::{Operation, Selected}; + +use crate::ptr; +use crate::sync::atomic::{AtomicBool, Ordering}; +use crate::sync::Mutex; + +/// Represents a thread blocked on a specific channel operation. +pub(crate) struct Entry { + /// The operation. + pub(crate) oper: Operation, + + /// Optional packet. + pub(crate) packet: *mut (), + + /// Context associated with the thread owning this operation. + pub(crate) cx: Context, +} + +/// A queue of threads blocked on channel operations. +/// +/// This data structure is used by threads to register blocking operations and get woken up once +/// an operation becomes ready. +pub(crate) struct Waker { + /// A list of select operations. + selectors: Vec<Entry>, + + /// A list of operations waiting to be ready. + observers: Vec<Entry>, +} + +impl Waker { + /// Creates a new `Waker`. + #[inline] + pub(crate) fn new() -> Self { + Waker { selectors: Vec::new(), observers: Vec::new() } + } + + /// Registers a select operation. + #[inline] + pub(crate) fn register(&mut self, oper: Operation, cx: &Context) { + self.register_with_packet(oper, ptr::null_mut(), cx); + } + + /// Registers a select operation and a packet. + #[inline] + pub(crate) fn register_with_packet(&mut self, oper: Operation, packet: *mut (), cx: &Context) { + self.selectors.push(Entry { oper, packet, cx: cx.clone() }); + } + + /// Unregisters a select operation. + #[inline] + pub(crate) fn unregister(&mut self, oper: Operation) -> Option<Entry> { + if let Some((i, _)) = + self.selectors.iter().enumerate().find(|&(_, entry)| entry.oper == oper) + { + let entry = self.selectors.remove(i); + Some(entry) + } else { + None + } + } + + /// Attempts to find another thread's entry, select the operation, and wake it up. + #[inline] + pub(crate) fn try_select(&mut self) -> Option<Entry> { + self.selectors + .iter() + .position(|selector| { + // Does the entry belong to a different thread? + selector.cx.thread_id() != current_thread_id() + && selector // Try selecting this operation. + .cx + .try_select(Selected::Operation(selector.oper)) + .is_ok() + && { + // Provide the packet. + selector.cx.store_packet(selector.packet); + // Wake the thread up. + selector.cx.unpark(); + true + } + }) + // Remove the entry from the queue to keep it clean and improve + // performance. + .map(|pos| self.selectors.remove(pos)) + } + + /// Notifies all operations waiting to be ready. + #[inline] + pub(crate) fn notify(&mut self) { + for entry in self.observers.drain(..) { + if entry.cx.try_select(Selected::Operation(entry.oper)).is_ok() { + entry.cx.unpark(); + } + } + } + + /// Notifies all registered operations that the channel is disconnected. + #[inline] + pub(crate) fn disconnect(&mut self) { + for entry in self.selectors.iter() { + if entry.cx.try_select(Selected::Disconnected).is_ok() { + // Wake the thread up. + // + // Here we don't remove the entry from the queue. Registered threads must + // unregister from the waker by themselves. They might also want to recover the + // packet value and destroy it, if necessary. + entry.cx.unpark(); + } + } + + self.notify(); + } +} + +impl Drop for Waker { + #[inline] + fn drop(&mut self) { + debug_assert_eq!(self.selectors.len(), 0); + debug_assert_eq!(self.observers.len(), 0); + } +} + +/// A waker that can be shared among threads without locking. +/// +/// This is a simple wrapper around `Waker` that internally uses a mutex for synchronization. +pub(crate) struct SyncWaker { + /// The inner `Waker`. + inner: Mutex<Waker>, + + /// `true` if the waker is empty. + is_empty: AtomicBool, +} + +impl SyncWaker { + /// Creates a new `SyncWaker`. + #[inline] + pub(crate) fn new() -> Self { + SyncWaker { inner: Mutex::new(Waker::new()), is_empty: AtomicBool::new(true) } + } + + /// Registers the current thread with an operation. + #[inline] + pub(crate) fn register(&self, oper: Operation, cx: &Context) { + let mut inner = self.inner.lock().unwrap(); + inner.register(oper, cx); + self.is_empty + .store(inner.selectors.is_empty() && inner.observers.is_empty(), Ordering::SeqCst); + } + + /// Unregisters an operation previously registered by the current thread. + #[inline] + pub(crate) fn unregister(&self, oper: Operation) -> Option<Entry> { + let mut inner = self.inner.lock().unwrap(); + let entry = inner.unregister(oper); + self.is_empty + .store(inner.selectors.is_empty() && inner.observers.is_empty(), Ordering::SeqCst); + entry + } + + /// Attempts to find one thread (not the current one), select its operation, and wake it up. + #[inline] + pub(crate) fn notify(&self) { + if !self.is_empty.load(Ordering::SeqCst) { + let mut inner = self.inner.lock().unwrap(); + if !self.is_empty.load(Ordering::SeqCst) { + inner.try_select(); + inner.notify(); + self.is_empty.store( + inner.selectors.is_empty() && inner.observers.is_empty(), + Ordering::SeqCst, + ); + } + } + } + + /// Notifies all threads that the channel is disconnected. + #[inline] + pub(crate) fn disconnect(&self) { + let mut inner = self.inner.lock().unwrap(); + inner.disconnect(); + self.is_empty + .store(inner.selectors.is_empty() && inner.observers.is_empty(), Ordering::SeqCst); + } +} + +impl Drop for SyncWaker { + #[inline] + fn drop(&mut self) { + debug_assert!(self.is_empty.load(Ordering::SeqCst)); + } +} + +/// Returns a unique id for the current thread. +#[inline] +pub fn current_thread_id() -> usize { + // `u8` is not drop so this variable will be available during thread destruction, + // whereas `thread::current()` would not be + thread_local! { static DUMMY: u8 = 0 } + DUMMY.with(|x| (x as *const u8).addr()) +} diff --git a/library/std/src/sync/mpmc/zero.rs b/library/std/src/sync/mpmc/zero.rs new file mode 100644 index 00000000000..fccd6c29a7e --- /dev/null +++ b/library/std/src/sync/mpmc/zero.rs @@ -0,0 +1,318 @@ +//! Zero-capacity channel. +//! +//! This kind of channel is also known as *rendezvous* channel. + +use super::context::Context; +use super::error::*; +use super::select::{Operation, Selected, Token}; +use super::utils::Backoff; +use super::waker::Waker; + +use crate::cell::UnsafeCell; +use crate::marker::PhantomData; +use crate::sync::atomic::{AtomicBool, Ordering}; +use crate::sync::Mutex; +use crate::time::Instant; +use crate::{fmt, ptr}; + +/// A pointer to a packet. +pub(crate) struct ZeroToken(*mut ()); + +impl Default for ZeroToken { + fn default() -> Self { + Self(ptr::null_mut()) + } +} + +impl fmt::Debug for ZeroToken { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Debug::fmt(&(self.0 as usize), f) + } +} + +/// A slot for passing one message from a sender to a receiver. +struct Packet<T> { + /// Equals `true` if the packet is allocated on the stack. + on_stack: bool, + + /// Equals `true` once the packet is ready for reading or writing. + ready: AtomicBool, + + /// The message. + msg: UnsafeCell<Option<T>>, +} + +impl<T> Packet<T> { + /// Creates an empty packet on the stack. + fn empty_on_stack() -> Packet<T> { + Packet { on_stack: true, ready: AtomicBool::new(false), msg: UnsafeCell::new(None) } + } + + /// Creates a packet on the stack, containing a message. + fn message_on_stack(msg: T) -> Packet<T> { + Packet { on_stack: true, ready: AtomicBool::new(false), msg: UnsafeCell::new(Some(msg)) } + } + + /// Waits until the packet becomes ready for reading or writing. + fn wait_ready(&self) { + let backoff = Backoff::new(); + while !self.ready.load(Ordering::Acquire) { + backoff.snooze(); + } + } +} + +/// Inner representation of a zero-capacity channel. +struct Inner { + /// Senders waiting to pair up with a receive operation. + senders: Waker, + + /// Receivers waiting to pair up with a send operation. + receivers: Waker, + + /// Equals `true` when the channel is disconnected. + is_disconnected: bool, +} + +/// Zero-capacity channel. +pub(crate) struct Channel<T> { + /// Inner representation of the channel. + inner: Mutex<Inner>, + + /// Indicates that dropping a `Channel<T>` may drop values of type `T`. + _marker: PhantomData<T>, +} + +impl<T> Channel<T> { + /// Constructs a new zero-capacity channel. + pub(crate) fn new() -> Self { + Channel { + inner: Mutex::new(Inner { + senders: Waker::new(), + receivers: Waker::new(), + is_disconnected: false, + }), + _marker: PhantomData, + } + } + + /// Writes a message into the packet. + pub(crate) unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> { + // If there is no packet, the channel is disconnected. + if token.zero.0.is_null() { + return Err(msg); + } + + let packet = &*(token.zero.0 as *const Packet<T>); + packet.msg.get().write(Some(msg)); + packet.ready.store(true, Ordering::Release); + Ok(()) + } + + /// Reads a message from the packet. + pub(crate) unsafe fn read(&self, token: &mut Token) -> Result<T, ()> { + // If there is no packet, the channel is disconnected. + if token.zero.0.is_null() { + return Err(()); + } + + let packet = &*(token.zero.0 as *const Packet<T>); + + if packet.on_stack { + // The message has been in the packet from the beginning, so there is no need to wait + // for it. However, after reading the message, we need to set `ready` to `true` in + // order to signal that the packet can be destroyed. + let msg = packet.msg.get().replace(None).unwrap(); + packet.ready.store(true, Ordering::Release); + Ok(msg) + } else { + // Wait until the message becomes available, then read it and destroy the + // heap-allocated packet. + packet.wait_ready(); + let msg = packet.msg.get().replace(None).unwrap(); + drop(Box::from_raw(token.zero.0 as *mut Packet<T>)); + Ok(msg) + } + } + + /// Attempts to send a message into the channel. + pub(crate) fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> { + let token = &mut Token::default(); + let mut inner = self.inner.lock().unwrap(); + + // If there's a waiting receiver, pair up with it. + if let Some(operation) = inner.receivers.try_select() { + token.zero.0 = operation.packet; + drop(inner); + unsafe { + self.write(token, msg).ok().unwrap(); + } + Ok(()) + } else if inner.is_disconnected { + Err(TrySendError::Disconnected(msg)) + } else { + Err(TrySendError::Full(msg)) + } + } + + /// Sends a message into the channel. + pub(crate) fn send( + &self, + msg: T, + deadline: Option<Instant>, + ) -> Result<(), SendTimeoutError<T>> { + let token = &mut Token::default(); + let mut inner = self.inner.lock().unwrap(); + + // If there's a waiting receiver, pair up with it. + if let Some(operation) = inner.receivers.try_select() { + token.zero.0 = operation.packet; + drop(inner); + unsafe { + self.write(token, msg).ok().unwrap(); + } + return Ok(()); + } + + if inner.is_disconnected { + return Err(SendTimeoutError::Disconnected(msg)); + } + + Context::with(|cx| { + // Prepare for blocking until a receiver wakes us up. + let oper = Operation::hook(token); + let mut packet = Packet::<T>::message_on_stack(msg); + inner.senders.register_with_packet(oper, &mut packet as *mut Packet<T> as *mut (), cx); + inner.receivers.notify(); + drop(inner); + + // Block the current thread. + let sel = cx.wait_until(deadline); + + match sel { + Selected::Waiting => unreachable!(), + Selected::Aborted => { + self.inner.lock().unwrap().senders.unregister(oper).unwrap(); + let msg = unsafe { packet.msg.get().replace(None).unwrap() }; + Err(SendTimeoutError::Timeout(msg)) + } + Selected::Disconnected => { + self.inner.lock().unwrap().senders.unregister(oper).unwrap(); + let msg = unsafe { packet.msg.get().replace(None).unwrap() }; + Err(SendTimeoutError::Disconnected(msg)) + } + Selected::Operation(_) => { + // Wait until the message is read, then drop the packet. + packet.wait_ready(); + Ok(()) + } + } + }) + } + + /// Attempts to receive a message without blocking. + pub(crate) fn try_recv(&self) -> Result<T, TryRecvError> { + let token = &mut Token::default(); + let mut inner = self.inner.lock().unwrap(); + + // If there's a waiting sender, pair up with it. + if let Some(operation) = inner.senders.try_select() { + token.zero.0 = operation.packet; + drop(inner); + unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) } + } else if inner.is_disconnected { + Err(TryRecvError::Disconnected) + } else { + Err(TryRecvError::Empty) + } + } + + /// Receives a message from the channel. + pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> { + let token = &mut Token::default(); + let mut inner = self.inner.lock().unwrap(); + + // If there's a waiting sender, pair up with it. + if let Some(operation) = inner.senders.try_select() { + token.zero.0 = operation.packet; + drop(inner); + unsafe { + return self.read(token).map_err(|_| RecvTimeoutError::Disconnected); + } + } + + if inner.is_disconnected { + return Err(RecvTimeoutError::Disconnected); + } + + Context::with(|cx| { + // Prepare for blocking until a sender wakes us up. + let oper = Operation::hook(token); + let mut packet = Packet::<T>::empty_on_stack(); + inner.receivers.register_with_packet( + oper, + &mut packet as *mut Packet<T> as *mut (), + cx, + ); + inner.senders.notify(); + drop(inner); + + // Block the current thread. + let sel = cx.wait_until(deadline); + + match sel { + Selected::Waiting => unreachable!(), + Selected::Aborted => { + self.inner.lock().unwrap().receivers.unregister(oper).unwrap(); + Err(RecvTimeoutError::Timeout) + } + Selected::Disconnected => { + self.inner.lock().unwrap().receivers.unregister(oper).unwrap(); + Err(RecvTimeoutError::Disconnected) + } + Selected::Operation(_) => { + // Wait until the message is provided, then read it. + packet.wait_ready(); + unsafe { Ok(packet.msg.get().replace(None).unwrap()) } + } + } + }) + } + + /// Disconnects the channel and wakes up all blocked senders and receivers. + /// + /// Returns `true` if this call disconnected the channel. + pub(crate) fn disconnect(&self) -> bool { + let mut inner = self.inner.lock().unwrap(); + + if !inner.is_disconnected { + inner.is_disconnected = true; + inner.senders.disconnect(); + inner.receivers.disconnect(); + true + } else { + false + } + } + + /// Returns the current number of messages inside the channel. + pub(crate) fn len(&self) -> usize { + 0 + } + + /// Returns the capacity of the channel. + #[allow(clippy::unnecessary_wraps)] // This is intentional. + pub(crate) fn capacity(&self) -> Option<usize> { + Some(0) + } + + /// Returns `true` if the channel is empty. + pub(crate) fn is_empty(&self) -> bool { + true + } + + /// Returns `true` if the channel is full. + pub(crate) fn is_full(&self) -> bool { + true + } +} diff --git a/library/std/src/sync/mpsc/blocking.rs b/library/std/src/sync/mpsc/blocking.rs deleted file mode 100644 index 021df7b096c..00000000000 --- a/library/std/src/sync/mpsc/blocking.rs +++ /dev/null @@ -1,82 +0,0 @@ -//! Generic support for building blocking abstractions. - -use crate::sync::atomic::{AtomicBool, Ordering}; -use crate::sync::Arc; -use crate::thread::{self, Thread}; -use crate::time::Instant; - -struct Inner { - thread: Thread, - woken: AtomicBool, -} - -unsafe impl Send for Inner {} -unsafe impl Sync for Inner {} - -#[derive(Clone)] -pub struct SignalToken { - inner: Arc<Inner>, -} - -pub struct WaitToken { - inner: Arc<Inner>, -} - -impl !Send for WaitToken {} - -impl !Sync for WaitToken {} - -pub fn tokens() -> (WaitToken, SignalToken) { - let inner = Arc::new(Inner { thread: thread::current(), woken: AtomicBool::new(false) }); - let wait_token = WaitToken { inner: inner.clone() }; - let signal_token = SignalToken { inner }; - (wait_token, signal_token) -} - -impl SignalToken { - pub fn signal(&self) -> bool { - let wake = self - .inner - .woken - .compare_exchange(false, true, Ordering::SeqCst, Ordering::SeqCst) - .is_ok(); - if wake { - self.inner.thread.unpark(); - } - wake - } - - /// Converts to an unsafe raw pointer. Useful for storing in a pipe's state - /// flag. - #[inline] - pub unsafe fn to_raw(self) -> *mut u8 { - Arc::into_raw(self.inner) as *mut u8 - } - - /// Converts from an unsafe raw pointer. Useful for retrieving a pipe's state - /// flag. - #[inline] - pub unsafe fn from_raw(signal_ptr: *mut u8) -> SignalToken { - SignalToken { inner: Arc::from_raw(signal_ptr as *mut Inner) } - } -} - -impl WaitToken { - pub fn wait(self) { - while !self.inner.woken.load(Ordering::SeqCst) { - thread::park() - } - } - - /// Returns `true` if we wake up normally. - pub fn wait_max_until(self, end: Instant) -> bool { - while !self.inner.woken.load(Ordering::SeqCst) { - let now = Instant::now(); - if now >= end { - return false; - } - thread::park_timeout(end - now) - } - true - } -} diff --git a/library/std/src/sync/mpsc/cache_aligned.rs b/library/std/src/sync/mpsc/cache_aligned.rs deleted file mode 100644 index 9197f0d6e6c..00000000000 --- a/library/std/src/sync/mpsc/cache_aligned.rs +++ /dev/null @@ -1,25 +0,0 @@ -use crate::ops::{Deref, DerefMut}; - -#[derive(Copy, Clone, Default, PartialEq, Eq, PartialOrd, Ord, Hash)] -#[cfg_attr(target_arch = "aarch64", repr(align(128)))] -#[cfg_attr(not(target_arch = "aarch64"), repr(align(64)))] -pub(super) struct CacheAligned<T>(pub T); - -impl<T> Deref for CacheAligned<T> { - type Target = T; - fn deref(&self) -> &Self::Target { - &self.0 - } -} - -impl<T> DerefMut for CacheAligned<T> { - fn deref_mut(&mut self) -> &mut Self::Target { - &mut self.0 - } -} - -impl<T> CacheAligned<T> { - pub(super) fn new(t: T) -> Self { - CacheAligned(t) - } -} diff --git a/library/std/src/sync/mpsc/mod.rs b/library/std/src/sync/mpsc/mod.rs index e85a8723965..27fba761ada 100644 --- a/library/std/src/sync/mpsc/mod.rs +++ b/library/std/src/sync/mpsc/mod.rs @@ -143,175 +143,16 @@ mod tests; #[cfg(all(test, not(target_os = "emscripten")))] mod sync_tests; -// A description of how Rust's channel implementation works -// -// Channels are supposed to be the basic building block for all other -// concurrent primitives that are used in Rust. As a result, the channel type -// needs to be highly optimized, flexible, and broad enough for use everywhere. -// -// The choice of implementation of all channels is to be built on lock-free data -// structures. The channels themselves are then consequently also lock-free data -// structures. As always with lock-free code, this is a very "here be dragons" -// territory, especially because I'm unaware of any academic papers that have -// gone into great length about channels of these flavors. -// -// ## Flavors of channels -// -// From the perspective of a consumer of this library, there is only one flavor -// of channel. This channel can be used as a stream and cloned to allow multiple -// senders. Under the hood, however, there are actually three flavors of -// channels in play. -// -// * Flavor::Oneshots - these channels are highly optimized for the one-send use -// case. They contain as few atomics as possible and -// involve one and exactly one allocation. -// * Streams - these channels are optimized for the non-shared use case. They -// use a different concurrent queue that is more tailored for this -// use case. The initial allocation of this flavor of channel is not -// optimized. -// * Shared - this is the most general form of channel that this module offers, -// a channel with multiple senders. This type is as optimized as it -// can be, but the previous two types mentioned are much faster for -// their use-cases. -// -// ## Concurrent queues -// -// The basic idea of Rust's Sender/Receiver types is that send() never blocks, -// but recv() obviously blocks. This means that under the hood there must be -// some shared and concurrent queue holding all of the actual data. -// -// With two flavors of channels, two flavors of queues are also used. We have -// chosen to use queues from a well-known author that are abbreviated as SPSC -// and MPSC (single producer, single consumer and multiple producer, single -// consumer). SPSC queues are used for streams while MPSC queues are used for -// shared channels. -// -// ### SPSC optimizations -// -// The SPSC queue found online is essentially a linked list of nodes where one -// half of the nodes are the "queue of data" and the other half of nodes are a -// cache of unused nodes. The unused nodes are used such that an allocation is -// not required on every push() and a free doesn't need to happen on every -// pop(). -// -// As found online, however, the cache of nodes is of an infinite size. This -// means that if a channel at one point in its life had 50k items in the queue, -// then the queue will always have the capacity for 50k items. I believed that -// this was an unnecessary limitation of the implementation, so I have altered -// the queue to optionally have a bound on the cache size. -// -// By default, streams will have an unbounded SPSC queue with a small-ish cache -// size. The hope is that the cache is still large enough to have very fast -// send() operations while not too large such that millions of channels can -// coexist at once. -// -// ### MPSC optimizations -// -// Right now the MPSC queue has not been optimized. Like the SPSC queue, it uses -// a linked list under the hood to earn its unboundedness, but I have not put -// forth much effort into having a cache of nodes similar to the SPSC queue. -// -// For now, I believe that this is "ok" because shared channels are not the most -// common type, but soon we may wish to revisit this queue choice and determine -// another candidate for backend storage of shared channels. -// -// ## Overview of the Implementation -// -// Now that there's a little background on the concurrent queues used, it's -// worth going into much more detail about the channels themselves. The basic -// pseudocode for a send/recv are: -// -// -// send(t) recv() -// queue.push(t) return if queue.pop() -// if increment() == -1 deschedule { -// wakeup() if decrement() > 0 -// cancel_deschedule() -// } -// queue.pop() -// -// As mentioned before, there are no locks in this implementation, only atomic -// instructions are used. -// -// ### The internal atomic counter -// -// Every channel has a shared counter with each half to keep track of the size -// of the queue. This counter is used to abort descheduling by the receiver and -// to know when to wake up on the sending side. -// -// As seen in the pseudocode, senders will increment this count and receivers -// will decrement the count. The theory behind this is that if a sender sees a -// -1 count, it will wake up the receiver, and if the receiver sees a 1+ count, -// then it doesn't need to block. -// -// The recv() method has a beginning call to pop(), and if successful, it needs -// to decrement the count. It is a crucial implementation detail that this -// decrement does *not* happen to the shared counter. If this were the case, -// then it would be possible for the counter to be very negative when there were -// no receivers waiting, in which case the senders would have to determine when -// it was actually appropriate to wake up a receiver. -// -// Instead, the "steal count" is kept track of separately (not atomically -// because it's only used by receivers), and then the decrement() call when -// descheduling will lump in all of the recent steals into one large decrement. -// -// The implication of this is that if a sender sees a -1 count, then there's -// guaranteed to be a waiter waiting! -// -// ## Native Implementation -// -// A major goal of these channels is to work seamlessly on and off the runtime. -// All of the previous race conditions have been worded in terms of -// scheduler-isms (which is obviously not available without the runtime). -// -// For now, native usage of channels (off the runtime) will fall back onto -// mutexes/cond vars for descheduling/atomic decisions. The no-contention path -// is still entirely lock-free, the "deschedule" blocks above are surrounded by -// a mutex and the "wakeup" blocks involve grabbing a mutex and signaling on a -// condition variable. -// -// ## Select -// -// Being able to support selection over channels has greatly influenced this -// design, and not only does selection need to work inside the runtime, but also -// outside the runtime. -// -// The implementation is fairly straightforward. The goal of select() is not to -// return some data, but only to return which channel can receive data without -// blocking. The implementation is essentially the entire blocking procedure -// followed by an increment as soon as its woken up. The cancellation procedure -// involves an increment and swapping out of to_wake to acquire ownership of the -// thread to unblock. -// -// Sadly this current implementation requires multiple allocations, so I have -// seen the throughput of select() be much worse than it should be. I do not -// believe that there is anything fundamental that needs to change about these -// channels, however, in order to support a more efficient select(). -// -// FIXME: Select is now removed, so these factors are ready to be cleaned up! -// -// # Conclusion -// -// And now that you've seen all the races that I found and attempted to fix, -// here's the code for you to find some more! - -use crate::cell::UnsafeCell; +// MPSC channels are built as a wrapper around MPMC channels, which +// were ported from the `crossbeam-channel` crate. MPMC channels are +// not exposed publicly, but if you are curious about the implementation, +// that's where everything is. + use crate::error; use crate::fmt; -use crate::mem; -use crate::sync::Arc; +use crate::sync::mpmc; use crate::time::{Duration, Instant}; -mod blocking; -mod mpsc_queue; -mod oneshot; -mod shared; -mod spsc_queue; -mod stream; -mod sync; - -mod cache_aligned; - /// The receiving half of Rust's [`channel`] (or [`sync_channel`]) type. /// This half can only be owned by one thread. /// @@ -341,7 +182,7 @@ mod cache_aligned; #[stable(feature = "rust1", since = "1.0.0")] #[cfg_attr(not(test), rustc_diagnostic_item = "Receiver")] pub struct Receiver<T> { - inner: UnsafeCell<Flavor<T>>, + inner: mpmc::Receiver<T>, } // The receiver port can be sent from place to place, so long as it @@ -498,7 +339,7 @@ pub struct IntoIter<T> { /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub struct Sender<T> { - inner: UnsafeCell<Flavor<T>>, + inner: mpmc::Sender<T>, } // The send port can be sent from place to place, so long as it @@ -557,7 +398,7 @@ impl<T> !Sync for Sender<T> {} /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub struct SyncSender<T> { - inner: Arc<sync::Packet<T>>, + inner: mpmc::Sender<T>, } #[stable(feature = "rust1", since = "1.0.0")] @@ -643,34 +484,6 @@ pub enum TrySendError<T> { Disconnected(#[stable(feature = "rust1", since = "1.0.0")] T), } -enum Flavor<T> { - Oneshot(Arc<oneshot::Packet<T>>), - Stream(Arc<stream::Packet<T>>), - Shared(Arc<shared::Packet<T>>), - Sync(Arc<sync::Packet<T>>), -} - -#[doc(hidden)] -trait UnsafeFlavor<T> { - fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>>; - unsafe fn inner_mut(&self) -> &mut Flavor<T> { - &mut *self.inner_unsafe().get() - } - unsafe fn inner(&self) -> &Flavor<T> { - &*self.inner_unsafe().get() - } -} -impl<T> UnsafeFlavor<T> for Sender<T> { - fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>> { - &self.inner - } -} -impl<T> UnsafeFlavor<T> for Receiver<T> { - fn inner_unsafe(&self) -> &UnsafeCell<Flavor<T>> { - &self.inner - } -} - /// Creates a new asynchronous channel, returning the sender/receiver halves. /// All data sent on the [`Sender`] will become available on the [`Receiver`] in /// the same order as it was sent, and no [`send`] will block the calling thread @@ -711,8 +524,8 @@ impl<T> UnsafeFlavor<T> for Receiver<T> { #[must_use] #[stable(feature = "rust1", since = "1.0.0")] pub fn channel<T>() -> (Sender<T>, Receiver<T>) { - let a = Arc::new(oneshot::Packet::new()); - (Sender::new(Flavor::Oneshot(a.clone())), Receiver::new(Flavor::Oneshot(a))) + let (tx, rx) = mpmc::channel(); + (Sender { inner: tx }, Receiver { inner: rx }) } /// Creates a new synchronous, bounded channel. @@ -760,8 +573,8 @@ pub fn channel<T>() -> (Sender<T>, Receiver<T>) { #[must_use] #[stable(feature = "rust1", since = "1.0.0")] pub fn sync_channel<T>(bound: usize) -> (SyncSender<T>, Receiver<T>) { - let a = Arc::new(sync::Packet::new(bound)); - (SyncSender::new(a.clone()), Receiver::new(Flavor::Sync(a))) + let (tx, rx) = mpmc::sync_channel(bound); + (SyncSender { inner: tx }, Receiver { inner: rx }) } //////////////////////////////////////////////////////////////////////////////// @@ -769,10 +582,6 @@ pub fn sync_channel<T>(bound: usize) -> (SyncSender<T>, Receiver<T>) { //////////////////////////////////////////////////////////////////////////////// impl<T> Sender<T> { - fn new(inner: Flavor<T>) -> Sender<T> { - Sender { inner: UnsafeCell::new(inner) } - } - /// Attempts to send a value on this channel, returning it back if it could /// not be sent. /// @@ -802,40 +611,7 @@ impl<T> Sender<T> { /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn send(&self, t: T) -> Result<(), SendError<T>> { - let (new_inner, ret) = match *unsafe { self.inner() } { - Flavor::Oneshot(ref p) => { - if !p.sent() { - return p.send(t).map_err(SendError); - } else { - let a = Arc::new(stream::Packet::new()); - let rx = Receiver::new(Flavor::Stream(a.clone())); - match p.upgrade(rx) { - oneshot::UpSuccess => { - let ret = a.send(t); - (a, ret) - } - oneshot::UpDisconnected => (a, Err(t)), - oneshot::UpWoke(token) => { - // This send cannot panic because the thread is - // asleep (we're looking at it), so the receiver - // can't go away. - a.send(t).ok().unwrap(); - token.signal(); - (a, Ok(())) - } - } - } - } - Flavor::Stream(ref p) => return p.send(t).map_err(SendError), - Flavor::Shared(ref p) => return p.send(t).map_err(SendError), - Flavor::Sync(..) => unreachable!(), - }; - - unsafe { - let tmp = Sender::new(Flavor::Stream(new_inner)); - mem::swap(self.inner_mut(), tmp.inner_mut()); - } - ret.map_err(SendError) + self.inner.send(t) } } @@ -847,57 +623,14 @@ impl<T> Clone for Sender<T> { /// (including the original) need to be dropped in order for /// [`Receiver::recv`] to stop blocking. fn clone(&self) -> Sender<T> { - let packet = match *unsafe { self.inner() } { - Flavor::Oneshot(ref p) => { - let a = Arc::new(shared::Packet::new()); - { - let guard = a.postinit_lock(); - let rx = Receiver::new(Flavor::Shared(a.clone())); - let sleeper = match p.upgrade(rx) { - oneshot::UpSuccess | oneshot::UpDisconnected => None, - oneshot::UpWoke(task) => Some(task), - }; - a.inherit_blocker(sleeper, guard); - } - a - } - Flavor::Stream(ref p) => { - let a = Arc::new(shared::Packet::new()); - { - let guard = a.postinit_lock(); - let rx = Receiver::new(Flavor::Shared(a.clone())); - let sleeper = match p.upgrade(rx) { - stream::UpSuccess | stream::UpDisconnected => None, - stream::UpWoke(task) => Some(task), - }; - a.inherit_blocker(sleeper, guard); - } - a - } - Flavor::Shared(ref p) => { - p.clone_chan(); - return Sender::new(Flavor::Shared(p.clone())); - } - Flavor::Sync(..) => unreachable!(), - }; - - unsafe { - let tmp = Sender::new(Flavor::Shared(packet.clone())); - mem::swap(self.inner_mut(), tmp.inner_mut()); - } - Sender::new(Flavor::Shared(packet)) + Sender { inner: self.inner.clone() } } } #[stable(feature = "rust1", since = "1.0.0")] impl<T> Drop for Sender<T> { fn drop(&mut self) { - match *unsafe { self.inner() } { - Flavor::Oneshot(ref p) => p.drop_chan(), - Flavor::Stream(ref p) => p.drop_chan(), - Flavor::Shared(ref p) => p.drop_chan(), - Flavor::Sync(..) => unreachable!(), - } + let _ = self.inner; } } @@ -913,10 +646,6 @@ impl<T> fmt::Debug for Sender<T> { //////////////////////////////////////////////////////////////////////////////// impl<T> SyncSender<T> { - fn new(inner: Arc<sync::Packet<T>>) -> SyncSender<T> { - SyncSender { inner } - } - /// Sends a value on this synchronous channel. /// /// This function will *block* until space in the internal buffer becomes @@ -955,7 +684,7 @@ impl<T> SyncSender<T> { /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn send(&self, t: T) -> Result<(), SendError<T>> { - self.inner.send(t).map_err(SendError) + self.inner.send(t) } /// Attempts to send a value on this channel without blocking. @@ -1016,15 +745,14 @@ impl<T> SyncSender<T> { #[stable(feature = "rust1", since = "1.0.0")] impl<T> Clone for SyncSender<T> { fn clone(&self) -> SyncSender<T> { - self.inner.clone_chan(); - SyncSender::new(self.inner.clone()) + SyncSender { inner: self.inner.clone() } } } #[stable(feature = "rust1", since = "1.0.0")] impl<T> Drop for SyncSender<T> { fn drop(&mut self) { - self.inner.drop_chan(); + let _ = self.inner; } } @@ -1040,10 +768,6 @@ impl<T> fmt::Debug for SyncSender<T> { //////////////////////////////////////////////////////////////////////////////// impl<T> Receiver<T> { - fn new(inner: Flavor<T>) -> Receiver<T> { - Receiver { inner: UnsafeCell::new(inner) } - } - /// Attempts to return a pending value on this receiver without blocking. /// /// This method will never block the caller in order to wait for data to @@ -1069,35 +793,7 @@ impl<T> Receiver<T> { /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn try_recv(&self) -> Result<T, TryRecvError> { - loop { - let new_port = match *unsafe { self.inner() } { - Flavor::Oneshot(ref p) => match p.try_recv() { - Ok(t) => return Ok(t), - Err(oneshot::Empty) => return Err(TryRecvError::Empty), - Err(oneshot::Disconnected) => return Err(TryRecvError::Disconnected), - Err(oneshot::Upgraded(rx)) => rx, - }, - Flavor::Stream(ref p) => match p.try_recv() { - Ok(t) => return Ok(t), - Err(stream::Empty) => return Err(TryRecvError::Empty), - Err(stream::Disconnected) => return Err(TryRecvError::Disconnected), - Err(stream::Upgraded(rx)) => rx, - }, - Flavor::Shared(ref p) => match p.try_recv() { - Ok(t) => return Ok(t), - Err(shared::Empty) => return Err(TryRecvError::Empty), - Err(shared::Disconnected) => return Err(TryRecvError::Disconnected), - }, - Flavor::Sync(ref p) => match p.try_recv() { - Ok(t) => return Ok(t), - Err(sync::Empty) => return Err(TryRecvError::Empty), - Err(sync::Disconnected) => return Err(TryRecvError::Disconnected), - }, - }; - unsafe { - mem::swap(self.inner_mut(), new_port.inner_mut()); - } - } + self.inner.try_recv() } /// Attempts to wait for a value on this receiver, returning an error if the @@ -1156,31 +852,7 @@ impl<T> Receiver<T> { /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub fn recv(&self) -> Result<T, RecvError> { - loop { - let new_port = match *unsafe { self.inner() } { - Flavor::Oneshot(ref p) => match p.recv(None) { - Ok(t) => return Ok(t), - Err(oneshot::Disconnected) => return Err(RecvError), - Err(oneshot::Upgraded(rx)) => rx, - Err(oneshot::Empty) => unreachable!(), - }, - Flavor::Stream(ref p) => match p.recv(None) { - Ok(t) => return Ok(t), - Err(stream::Disconnected) => return Err(RecvError), - Err(stream::Upgraded(rx)) => rx, - Err(stream::Empty) => unreachable!(), - }, - Flavor::Shared(ref p) => match p.recv(None) { - Ok(t) => return Ok(t), - Err(shared::Disconnected) => return Err(RecvError), - Err(shared::Empty) => unreachable!(), - }, - Flavor::Sync(ref p) => return p.recv(None).map_err(|_| RecvError), - }; - unsafe { - mem::swap(self.inner_mut(), new_port.inner_mut()); - } - } + self.inner.recv() } /// Attempts to wait for a value on this receiver, returning an error if the @@ -1198,34 +870,6 @@ impl<T> Receiver<T> { /// However, since channels are buffered, messages sent before the disconnect /// will still be properly received. /// - /// # Known Issues - /// - /// There is currently a known issue (see [`#39364`]) that causes `recv_timeout` - /// to panic unexpectedly with the following example: - /// - /// ```no_run - /// use std::sync::mpsc::channel; - /// use std::thread; - /// use std::time::Duration; - /// - /// let (tx, rx) = channel::<String>(); - /// - /// thread::spawn(move || { - /// let d = Duration::from_millis(10); - /// loop { - /// println!("recv"); - /// let _r = rx.recv_timeout(d); - /// } - /// }); - /// - /// thread::sleep(Duration::from_millis(100)); - /// let _c1 = tx.clone(); - /// - /// thread::sleep(Duration::from_secs(1)); - /// ``` - /// - /// [`#39364`]: https://github.com/rust-lang/rust/issues/39364 - /// /// # Examples /// /// Successfully receiving value before encountering timeout: @@ -1268,17 +912,7 @@ impl<T> Receiver<T> { /// ``` #[stable(feature = "mpsc_recv_timeout", since = "1.12.0")] pub fn recv_timeout(&self, timeout: Duration) -> Result<T, RecvTimeoutError> { - // Do an optimistic try_recv to avoid the performance impact of - // Instant::now() in the full-channel case. - match self.try_recv() { - Ok(result) => Ok(result), - Err(TryRecvError::Disconnected) => Err(RecvTimeoutError::Disconnected), - Err(TryRecvError::Empty) => match Instant::now().checked_add(timeout) { - Some(deadline) => self.recv_deadline(deadline), - // So far in the future that it's practically the same as waiting indefinitely. - None => self.recv().map_err(RecvTimeoutError::from), - }, - } + self.inner.recv_timeout(timeout) } /// Attempts to wait for a value on this receiver, returning an error if the @@ -1339,46 +973,7 @@ impl<T> Receiver<T> { /// ``` #[unstable(feature = "deadline_api", issue = "46316")] pub fn recv_deadline(&self, deadline: Instant) -> Result<T, RecvTimeoutError> { - use self::RecvTimeoutError::*; - - loop { - let port_or_empty = match *unsafe { self.inner() } { - Flavor::Oneshot(ref p) => match p.recv(Some(deadline)) { - Ok(t) => return Ok(t), - Err(oneshot::Disconnected) => return Err(Disconnected), - Err(oneshot::Upgraded(rx)) => Some(rx), - Err(oneshot::Empty) => None, - }, - Flavor::Stream(ref p) => match p.recv(Some(deadline)) { - Ok(t) => return Ok(t), - Err(stream::Disconnected) => return Err(Disconnected), - Err(stream::Upgraded(rx)) => Some(rx), - Err(stream::Empty) => None, - }, - Flavor::Shared(ref p) => match p.recv(Some(deadline)) { - Ok(t) => return Ok(t), - Err(shared::Disconnected) => return Err(Disconnected), - Err(shared::Empty) => None, - }, - Flavor::Sync(ref p) => match p.recv(Some(deadline)) { - Ok(t) => return Ok(t), - Err(sync::Disconnected) => return Err(Disconnected), - Err(sync::Empty) => None, - }, - }; - - if let Some(new_port) = port_or_empty { - unsafe { - mem::swap(self.inner_mut(), new_port.inner_mut()); - } - } - - // If we're already passed the deadline, and we're here without - // data, return a timeout, else try again. - if Instant::now() >= deadline { - return Err(Timeout); - } - } + self.inner.recv_deadline(deadline) } /// Returns an iterator that will block waiting for messages, but never @@ -1500,12 +1095,7 @@ impl<T> IntoIterator for Receiver<T> { #[stable(feature = "rust1", since = "1.0.0")] impl<T> Drop for Receiver<T> { fn drop(&mut self) { - match *unsafe { self.inner() } { - Flavor::Oneshot(ref p) => p.drop_port(), - Flavor::Stream(ref p) => p.drop_port(), - Flavor::Shared(ref p) => p.drop_port(), - Flavor::Sync(ref p) => p.drop_port(), - } + let _ = self.inner; } } diff --git a/library/std/src/sync/mpsc/mpsc_queue.rs b/library/std/src/sync/mpsc/mpsc_queue.rs deleted file mode 100644 index 7322512e3b4..00000000000 --- a/library/std/src/sync/mpsc/mpsc_queue.rs +++ /dev/null @@ -1,124 +0,0 @@ -//! A mostly lock-free multi-producer, single consumer queue. -//! -//! This module contains an implementation of a concurrent MPSC queue. This -//! queue can be used to share data between threads, and is also used as the -//! building block of channels in rust. -//! -//! Note that the current implementation of this queue has a caveat of the `pop` -//! method, and see the method for more information about it. Due to this -//! caveat, this queue might not be appropriate for all use-cases. - -// The original implementation is based off: -// https://www.1024cores.net/home/lock-free-algorithms/queues/non-intrusive-mpsc-node-based-queue -// -// Note that back when the code was imported, it was licensed under the BSD-2-Clause license: -// http://web.archive.org/web/20110411011612/https://www.1024cores.net/home/lock-free-algorithms/queues/unbounded-spsc-queue -// -// The original author of the code agreed to relicense it under `MIT OR Apache-2.0` in 2017, so as -// of today the license of this file is the same as the rest of the codebase: -// https://github.com/rust-lang/rust/pull/42149 - -#[cfg(all(test, not(target_os = "emscripten")))] -mod tests; - -pub use self::PopResult::*; - -use core::cell::UnsafeCell; -use core::ptr; - -use crate::boxed::Box; -use crate::sync::atomic::{AtomicPtr, Ordering}; - -/// A result of the `pop` function. -pub enum PopResult<T> { - /// Some data has been popped - Data(T), - /// The queue is empty - Empty, - /// The queue is in an inconsistent state. Popping data should succeed, but - /// some pushers have yet to make enough progress in order allow a pop to - /// succeed. It is recommended that a pop() occur "in the near future" in - /// order to see if the sender has made progress or not - Inconsistent, -} - -struct Node<T> { - next: AtomicPtr<Node<T>>, - value: Option<T>, -} - -/// The multi-producer single-consumer structure. This is not cloneable, but it -/// may be safely shared so long as it is guaranteed that there is only one -/// popper at a time (many pushers are allowed). -pub struct Queue<T> { - head: AtomicPtr<Node<T>>, - tail: UnsafeCell<*mut Node<T>>, -} - -unsafe impl<T: Send> Send for Queue<T> {} -unsafe impl<T: Send> Sync for Queue<T> {} - -impl<T> Node<T> { - unsafe fn new(v: Option<T>) -> *mut Node<T> { - Box::into_raw(box Node { next: AtomicPtr::new(ptr::null_mut()), value: v }) - } -} - -impl<T> Queue<T> { - /// Creates a new queue that is safe to share among multiple producers and - /// one consumer. - pub fn new() -> Queue<T> { - let stub = unsafe { Node::new(None) }; - Queue { head: AtomicPtr::new(stub), tail: UnsafeCell::new(stub) } - } - - /// Pushes a new value onto this queue. - pub fn push(&self, t: T) { - unsafe { - let n = Node::new(Some(t)); - let prev = self.head.swap(n, Ordering::AcqRel); - (*prev).next.store(n, Ordering::Release); - } - } - - /// Pops some data from this queue. - /// - /// Note that the current implementation means that this function cannot - /// return `Option<T>`. It is possible for this queue to be in an - /// inconsistent state where many pushes have succeeded and completely - /// finished, but pops cannot return `Some(t)`. This inconsistent state - /// happens when a pusher is pre-empted at an inopportune moment. - /// - /// This inconsistent state means that this queue does indeed have data, but - /// it does not currently have access to it at this time. - pub fn pop(&self) -> PopResult<T> { - unsafe { - let tail = *self.tail.get(); - let next = (*tail).next.load(Ordering::Acquire); - - if !next.is_null() { - *self.tail.get() = next; - assert!((*tail).value.is_none()); - assert!((*next).value.is_some()); - let ret = (*next).value.take().unwrap(); - let _: Box<Node<T>> = Box::from_raw(tail); - return Data(ret); - } - - if self.head.load(Ordering::Acquire) == tail { Empty } else { Inconsistent } - } - } -} - -impl<T> Drop for Queue<T> { - fn drop(&mut self) { - unsafe { - let mut cur = *self.tail.get(); - while !cur.is_null() { - let next = (*cur).next.load(Ordering::Relaxed); - let _: Box<Node<T>> = Box::from_raw(cur); - cur = next; - } - } - } -} diff --git a/library/std/src/sync/mpsc/mpsc_queue/tests.rs b/library/std/src/sync/mpsc/mpsc_queue/tests.rs deleted file mode 100644 index 34b2a9a98ac..00000000000 --- a/library/std/src/sync/mpsc/mpsc_queue/tests.rs +++ /dev/null @@ -1,47 +0,0 @@ -use super::{Data, Empty, Inconsistent, Queue}; -use crate::sync::mpsc::channel; -use crate::sync::Arc; -use crate::thread; - -#[test] -fn test_full() { - let q: Queue<Box<_>> = Queue::new(); - q.push(Box::new(1)); - q.push(Box::new(2)); -} - -#[test] -fn test() { - let nthreads = 8; - let nmsgs = if cfg!(miri) { 100 } else { 1000 }; - let q = Queue::new(); - match q.pop() { - Empty => {} - Inconsistent | Data(..) => panic!(), - } - let (tx, rx) = channel(); - let q = Arc::new(q); - - for _ in 0..nthreads { - let tx = tx.clone(); - let q = q.clone(); - thread::spawn(move || { - for i in 0..nmsgs { - q.push(i); - } - tx.send(()).unwrap(); - }); - } - - let mut i = 0; - while i < nthreads * nmsgs { - match q.pop() { - Empty | Inconsistent => {} - Data(_) => i += 1, - } - } - drop(tx); - for _ in 0..nthreads { - rx.recv().unwrap(); - } -} diff --git a/library/std/src/sync/mpsc/oneshot.rs b/library/std/src/sync/mpsc/oneshot.rs deleted file mode 100644 index 0e259b8aecb..00000000000 --- a/library/std/src/sync/mpsc/oneshot.rs +++ /dev/null @@ -1,315 +0,0 @@ -/// Oneshot channels/ports -/// -/// This is the initial flavor of channels/ports used for comm module. This is -/// an optimization for the one-use case of a channel. The major optimization of -/// this type is to have one and exactly one allocation when the chan/port pair -/// is created. -/// -/// Another possible optimization would be to not use an Arc box because -/// in theory we know when the shared packet can be deallocated (no real need -/// for the atomic reference counting), but I was having trouble how to destroy -/// the data early in a drop of a Port. -/// -/// # Implementation -/// -/// Oneshots are implemented around one atomic usize variable. This variable -/// indicates both the state of the port/chan but also contains any threads -/// blocked on the port. All atomic operations happen on this one word. -/// -/// In order to upgrade a oneshot channel, an upgrade is considered a disconnect -/// on behalf of the channel side of things (it can be mentally thought of as -/// consuming the port). This upgrade is then also stored in the shared packet. -/// The one caveat to consider is that when a port sees a disconnected channel -/// it must check for data because there is no "data plus upgrade" state. -pub use self::Failure::*; -use self::MyUpgrade::*; -pub use self::UpgradeResult::*; - -use crate::cell::UnsafeCell; -use crate::ptr; -use crate::sync::atomic::{AtomicPtr, Ordering}; -use crate::sync::mpsc::blocking::{self, SignalToken}; -use crate::sync::mpsc::Receiver; -use crate::time::Instant; - -// Various states you can find a port in. -const EMPTY: *mut u8 = ptr::invalid_mut::<u8>(0); // initial state: no data, no blocked receiver -const DATA: *mut u8 = ptr::invalid_mut::<u8>(1); // data ready for receiver to take -const DISCONNECTED: *mut u8 = ptr::invalid_mut::<u8>(2); // channel is disconnected OR upgraded -// Any other value represents a pointer to a SignalToken value. The -// protocol ensures that when the state moves *to* a pointer, -// ownership of the token is given to the packet, and when the state -// moves *from* a pointer, ownership of the token is transferred to -// whoever changed the state. - -pub struct Packet<T> { - // Internal state of the chan/port pair (stores the blocked thread as well) - state: AtomicPtr<u8>, - // One-shot data slot location - data: UnsafeCell<Option<T>>, - // when used for the second time, a oneshot channel must be upgraded, and - // this contains the slot for the upgrade - upgrade: UnsafeCell<MyUpgrade<T>>, -} - -pub enum Failure<T> { - Empty, - Disconnected, - Upgraded(Receiver<T>), -} - -pub enum UpgradeResult { - UpSuccess, - UpDisconnected, - UpWoke(SignalToken), -} - -enum MyUpgrade<T> { - NothingSent, - SendUsed, - GoUp(Receiver<T>), -} - -impl<T> Packet<T> { - pub fn new() -> Packet<T> { - Packet { - data: UnsafeCell::new(None), - upgrade: UnsafeCell::new(NothingSent), - state: AtomicPtr::new(EMPTY), - } - } - - pub fn send(&self, t: T) -> Result<(), T> { - unsafe { - // Sanity check - match *self.upgrade.get() { - NothingSent => {} - _ => panic!("sending on a oneshot that's already sent on "), - } - assert!((*self.data.get()).is_none()); - ptr::write(self.data.get(), Some(t)); - ptr::write(self.upgrade.get(), SendUsed); - - match self.state.swap(DATA, Ordering::SeqCst) { - // Sent the data, no one was waiting - EMPTY => Ok(()), - - // Couldn't send the data, the port hung up first. Return the data - // back up the stack. - DISCONNECTED => { - self.state.swap(DISCONNECTED, Ordering::SeqCst); - ptr::write(self.upgrade.get(), NothingSent); - Err((&mut *self.data.get()).take().unwrap()) - } - - // Not possible, these are one-use channels - DATA => unreachable!(), - - // There is a thread waiting on the other end. We leave the 'DATA' - // state inside so it'll pick it up on the other end. - ptr => { - SignalToken::from_raw(ptr).signal(); - Ok(()) - } - } - } - } - - // Just tests whether this channel has been sent on or not, this is only - // safe to use from the sender. - pub fn sent(&self) -> bool { - unsafe { !matches!(*self.upgrade.get(), NothingSent) } - } - - pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure<T>> { - // Attempt to not block the thread (it's a little expensive). If it looks - // like we're not empty, then immediately go through to `try_recv`. - if self.state.load(Ordering::SeqCst) == EMPTY { - let (wait_token, signal_token) = blocking::tokens(); - let ptr = unsafe { signal_token.to_raw() }; - - // race with senders to enter the blocking state - if self.state.compare_exchange(EMPTY, ptr, Ordering::SeqCst, Ordering::SeqCst).is_ok() { - if let Some(deadline) = deadline { - let timed_out = !wait_token.wait_max_until(deadline); - // Try to reset the state - if timed_out { - self.abort_selection().map_err(Upgraded)?; - } - } else { - wait_token.wait(); - debug_assert!(self.state.load(Ordering::SeqCst) != EMPTY); - } - } else { - // drop the signal token, since we never blocked - drop(unsafe { SignalToken::from_raw(ptr) }); - } - } - - self.try_recv() - } - - pub fn try_recv(&self) -> Result<T, Failure<T>> { - unsafe { - match self.state.load(Ordering::SeqCst) { - EMPTY => Err(Empty), - - // We saw some data on the channel, but the channel can be used - // again to send us an upgrade. As a result, we need to re-insert - // into the channel that there's no data available (otherwise we'll - // just see DATA next time). This is done as a cmpxchg because if - // the state changes under our feet we'd rather just see that state - // change. - DATA => { - let _ = self.state.compare_exchange( - DATA, - EMPTY, - Ordering::SeqCst, - Ordering::SeqCst, - ); - match (&mut *self.data.get()).take() { - Some(data) => Ok(data), - None => unreachable!(), - } - } - - // There's no guarantee that we receive before an upgrade happens, - // and an upgrade flags the channel as disconnected, so when we see - // this we first need to check if there's data available and *then* - // we go through and process the upgrade. - DISCONNECTED => match (&mut *self.data.get()).take() { - Some(data) => Ok(data), - None => match ptr::replace(self.upgrade.get(), SendUsed) { - SendUsed | NothingSent => Err(Disconnected), - GoUp(upgrade) => Err(Upgraded(upgrade)), - }, - }, - - // We are the sole receiver; there cannot be a blocking - // receiver already. - _ => unreachable!(), - } - } - } - - // Returns whether the upgrade was completed. If the upgrade wasn't - // completed, then the port couldn't get sent to the other half (it will - // never receive it). - pub fn upgrade(&self, up: Receiver<T>) -> UpgradeResult { - unsafe { - let prev = match *self.upgrade.get() { - NothingSent => NothingSent, - SendUsed => SendUsed, - _ => panic!("upgrading again"), - }; - ptr::write(self.upgrade.get(), GoUp(up)); - - match self.state.swap(DISCONNECTED, Ordering::SeqCst) { - // If the channel is empty or has data on it, then we're good to go. - // Senders will check the data before the upgrade (in case we - // plastered over the DATA state). - DATA | EMPTY => UpSuccess, - - // If the other end is already disconnected, then we failed the - // upgrade. Be sure to trash the port we were given. - DISCONNECTED => { - ptr::replace(self.upgrade.get(), prev); - UpDisconnected - } - - // If someone's waiting, we gotta wake them up - ptr => UpWoke(SignalToken::from_raw(ptr)), - } - } - } - - pub fn drop_chan(&self) { - match self.state.swap(DISCONNECTED, Ordering::SeqCst) { - DATA | DISCONNECTED | EMPTY => {} - - // If someone's waiting, we gotta wake them up - ptr => unsafe { - SignalToken::from_raw(ptr).signal(); - }, - } - } - - pub fn drop_port(&self) { - match self.state.swap(DISCONNECTED, Ordering::SeqCst) { - // An empty channel has nothing to do, and a remotely disconnected - // channel also has nothing to do b/c we're about to run the drop - // glue - DISCONNECTED | EMPTY => {} - - // There's data on the channel, so make sure we destroy it promptly. - // This is why not using an arc is a little difficult (need the box - // to stay valid while we take the data). - DATA => unsafe { - (&mut *self.data.get()).take().unwrap(); - }, - - // We're the only ones that can block on this port - _ => unreachable!(), - } - } - - //////////////////////////////////////////////////////////////////////////// - // select implementation - //////////////////////////////////////////////////////////////////////////// - - // Remove a previous selecting thread from this port. This ensures that the - // blocked thread will no longer be visible to any other threads. - // - // The return value indicates whether there's data on this port. - pub fn abort_selection(&self) -> Result<bool, Receiver<T>> { - let state = match self.state.load(Ordering::SeqCst) { - // Each of these states means that no further activity will happen - // with regard to abortion selection - s @ (EMPTY | DATA | DISCONNECTED) => s, - - // If we've got a blocked thread, then use an atomic to gain ownership - // of it (may fail) - ptr => self - .state - .compare_exchange(ptr, EMPTY, Ordering::SeqCst, Ordering::SeqCst) - .unwrap_or_else(|x| x), - }; - - // Now that we've got ownership of our state, figure out what to do - // about it. - match state { - EMPTY => unreachable!(), - // our thread used for select was stolen - DATA => Ok(true), - - // If the other end has hung up, then we have complete ownership - // of the port. First, check if there was data waiting for us. This - // is possible if the other end sent something and then hung up. - // - // We then need to check to see if there was an upgrade requested, - // and if so, the upgraded port needs to have its selection aborted. - DISCONNECTED => unsafe { - if (*self.data.get()).is_some() { - Ok(true) - } else { - match ptr::replace(self.upgrade.get(), SendUsed) { - GoUp(port) => Err(port), - _ => Ok(true), - } - } - }, - - // We woke ourselves up from select. - ptr => unsafe { - drop(SignalToken::from_raw(ptr)); - Ok(false) - }, - } - } -} - -impl<T> Drop for Packet<T> { - fn drop(&mut self) { - assert_eq!(self.state.load(Ordering::SeqCst), DISCONNECTED); - } -} diff --git a/library/std/src/sync/mpsc/shared.rs b/library/std/src/sync/mpsc/shared.rs deleted file mode 100644 index 51917bd96bd..00000000000 --- a/library/std/src/sync/mpsc/shared.rs +++ /dev/null @@ -1,501 +0,0 @@ -/// Shared channels. -/// -/// This is the flavor of channels which are not necessarily optimized for any -/// particular use case, but are the most general in how they are used. Shared -/// channels are cloneable allowing for multiple senders. -/// -/// High level implementation details can be found in the comment of the parent -/// module. You'll also note that the implementation of the shared and stream -/// channels are quite similar, and this is no coincidence! -pub use self::Failure::*; -use self::StartResult::*; - -use core::cmp; -use core::intrinsics::abort; - -use crate::cell::UnsafeCell; -use crate::ptr; -use crate::sync::atomic::{AtomicBool, AtomicIsize, AtomicPtr, AtomicUsize, Ordering}; -use crate::sync::mpsc::blocking::{self, SignalToken}; -use crate::sync::mpsc::mpsc_queue as mpsc; -use crate::sync::{Mutex, MutexGuard}; -use crate::thread; -use crate::time::Instant; - -const DISCONNECTED: isize = isize::MIN; -const FUDGE: isize = 1024; -const MAX_REFCOUNT: usize = (isize::MAX) as usize; -#[cfg(test)] -const MAX_STEALS: isize = 5; -#[cfg(not(test))] -const MAX_STEALS: isize = 1 << 20; -const EMPTY: *mut u8 = ptr::null_mut(); // initial state: no data, no blocked receiver - -pub struct Packet<T> { - queue: mpsc::Queue<T>, - cnt: AtomicIsize, // How many items are on this channel - steals: UnsafeCell<isize>, // How many times has a port received without blocking? - to_wake: AtomicPtr<u8>, // SignalToken for wake up - - // The number of channels which are currently using this packet. - channels: AtomicUsize, - - // See the discussion in Port::drop and the channel send methods for what - // these are used for - port_dropped: AtomicBool, - sender_drain: AtomicIsize, - - // this lock protects various portions of this implementation during - // select() - select_lock: Mutex<()>, -} - -pub enum Failure { - Empty, - Disconnected, -} - -#[derive(PartialEq, Eq)] -enum StartResult { - Installed, - Abort, -} - -impl<T> Packet<T> { - // Creation of a packet *must* be followed by a call to postinit_lock - // and later by inherit_blocker - pub fn new() -> Packet<T> { - Packet { - queue: mpsc::Queue::new(), - cnt: AtomicIsize::new(0), - steals: UnsafeCell::new(0), - to_wake: AtomicPtr::new(EMPTY), - channels: AtomicUsize::new(2), - port_dropped: AtomicBool::new(false), - sender_drain: AtomicIsize::new(0), - select_lock: Mutex::new(()), - } - } - - // This function should be used after newly created Packet - // was wrapped with an Arc - // In other case mutex data will be duplicated while cloning - // and that could cause problems on platforms where it is - // represented by opaque data structure - pub fn postinit_lock(&self) -> MutexGuard<'_, ()> { - self.select_lock.lock().unwrap() - } - - // This function is used at the creation of a shared packet to inherit a - // previously blocked thread. This is done to prevent spurious wakeups of - // threads in select(). - // - // This can only be called at channel-creation time - pub fn inherit_blocker(&self, token: Option<SignalToken>, guard: MutexGuard<'_, ()>) { - if let Some(token) = token { - assert_eq!(self.cnt.load(Ordering::SeqCst), 0); - assert_eq!(self.to_wake.load(Ordering::SeqCst), EMPTY); - self.to_wake.store(unsafe { token.to_raw() }, Ordering::SeqCst); - self.cnt.store(-1, Ordering::SeqCst); - - // This store is a little sketchy. What's happening here is that - // we're transferring a blocker from a oneshot or stream channel to - // this shared channel. In doing so, we never spuriously wake them - // up and rather only wake them up at the appropriate time. This - // implementation of shared channels assumes that any blocking - // recv() will undo the increment of steals performed in try_recv() - // once the recv is complete. This thread that we're inheriting, - // however, is not in the middle of recv. Hence, the first time we - // wake them up, they're going to wake up from their old port, move - // on to the upgraded port, and then call the block recv() function. - // - // When calling this function, they'll find there's data immediately - // available, counting it as a steal. This in fact wasn't a steal - // because we appropriately blocked them waiting for data. - // - // To offset this bad increment, we initially set the steal count to - // -1. You'll find some special code in abort_selection() as well to - // ensure that this -1 steal count doesn't escape too far. - unsafe { - *self.steals.get() = -1; - } - } - - // When the shared packet is constructed, we grabbed this lock. The - // purpose of this lock is to ensure that abort_selection() doesn't - // interfere with this method. After we unlock this lock, we're - // signifying that we're done modifying self.cnt and self.to_wake and - // the port is ready for the world to continue using it. - drop(guard); - } - - pub fn send(&self, t: T) -> Result<(), T> { - // See Port::drop for what's going on - if self.port_dropped.load(Ordering::SeqCst) { - return Err(t); - } - - // Note that the multiple sender case is a little trickier - // semantically than the single sender case. The logic for - // incrementing is "add and if disconnected store disconnected". - // This could end up leading some senders to believe that there - // wasn't a disconnect if in fact there was a disconnect. This means - // that while one thread is attempting to re-store the disconnected - // states, other threads could walk through merrily incrementing - // this very-negative disconnected count. To prevent senders from - // spuriously attempting to send when the channels is actually - // disconnected, the count has a ranged check here. - // - // This is also done for another reason. Remember that the return - // value of this function is: - // - // `true` == the data *may* be received, this essentially has no - // meaning - // `false` == the data will *never* be received, this has a lot of - // meaning - // - // In the SPSC case, we have a check of 'queue.is_empty()' to see - // whether the data was actually received, but this same condition - // means nothing in a multi-producer context. As a result, this - // preflight check serves as the definitive "this will never be - // received". Once we get beyond this check, we have permanently - // entered the realm of "this may be received" - if self.cnt.load(Ordering::SeqCst) < DISCONNECTED + FUDGE { - return Err(t); - } - - self.queue.push(t); - match self.cnt.fetch_add(1, Ordering::SeqCst) { - -1 => { - self.take_to_wake().signal(); - } - - // In this case, we have possibly failed to send our data, and - // we need to consider re-popping the data in order to fully - // destroy it. We must arbitrate among the multiple senders, - // however, because the queues that we're using are - // single-consumer queues. In order to do this, all exiting - // pushers will use an atomic count in order to count those - // flowing through. Pushers who see 0 are required to drain as - // much as possible, and then can only exit when they are the - // only pusher (otherwise they must try again). - n if n < DISCONNECTED + FUDGE => { - // see the comment in 'try' for a shared channel for why this - // window of "not disconnected" is ok. - self.cnt.store(DISCONNECTED, Ordering::SeqCst); - - if self.sender_drain.fetch_add(1, Ordering::SeqCst) == 0 { - loop { - // drain the queue, for info on the thread yield see the - // discussion in try_recv - loop { - match self.queue.pop() { - mpsc::Data(..) => {} - mpsc::Empty => break, - mpsc::Inconsistent => thread::yield_now(), - } - } - // maybe we're done, if we're not the last ones - // here, then we need to go try again. - if self.sender_drain.fetch_sub(1, Ordering::SeqCst) == 1 { - break; - } - } - - // At this point, there may still be data on the queue, - // but only if the count hasn't been incremented and - // some other sender hasn't finished pushing data just - // yet. That sender in question will drain its own data. - } - } - - // Can't make any assumptions about this case like in the SPSC case. - _ => {} - } - - Ok(()) - } - - pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure> { - // This code is essentially the exact same as that found in the stream - // case (see stream.rs) - match self.try_recv() { - Err(Empty) => {} - data => return data, - } - - let (wait_token, signal_token) = blocking::tokens(); - if self.decrement(signal_token) == Installed { - if let Some(deadline) = deadline { - let timed_out = !wait_token.wait_max_until(deadline); - if timed_out { - self.abort_selection(false); - } - } else { - wait_token.wait(); - } - } - - match self.try_recv() { - data @ Ok(..) => unsafe { - *self.steals.get() -= 1; - data - }, - data => data, - } - } - - // Essentially the exact same thing as the stream decrement function. - // Returns true if blocking should proceed. - fn decrement(&self, token: SignalToken) -> StartResult { - unsafe { - assert_eq!( - self.to_wake.load(Ordering::SeqCst), - EMPTY, - "This is a known bug in the Rust standard library. See https://github.com/rust-lang/rust/issues/39364" - ); - let ptr = token.to_raw(); - self.to_wake.store(ptr, Ordering::SeqCst); - - let steals = ptr::replace(self.steals.get(), 0); - - match self.cnt.fetch_sub(1 + steals, Ordering::SeqCst) { - DISCONNECTED => { - self.cnt.store(DISCONNECTED, Ordering::SeqCst); - } - // If we factor in our steals and notice that the channel has no - // data, we successfully sleep - n => { - assert!(n >= 0); - if n - steals <= 0 { - return Installed; - } - } - } - - self.to_wake.store(EMPTY, Ordering::SeqCst); - drop(SignalToken::from_raw(ptr)); - Abort - } - } - - pub fn try_recv(&self) -> Result<T, Failure> { - let ret = match self.queue.pop() { - mpsc::Data(t) => Some(t), - mpsc::Empty => None, - - // This is a bit of an interesting case. The channel is reported as - // having data available, but our pop() has failed due to the queue - // being in an inconsistent state. This means that there is some - // pusher somewhere which has yet to complete, but we are guaranteed - // that a pop will eventually succeed. In this case, we spin in a - // yield loop because the remote sender should finish their enqueue - // operation "very quickly". - // - // Avoiding this yield loop would require a different queue - // abstraction which provides the guarantee that after M pushes have - // succeeded, at least M pops will succeed. The current queues - // guarantee that if there are N active pushes, you can pop N times - // once all N have finished. - mpsc::Inconsistent => { - let data; - loop { - thread::yield_now(); - match self.queue.pop() { - mpsc::Data(t) => { - data = t; - break; - } - mpsc::Empty => panic!("inconsistent => empty"), - mpsc::Inconsistent => {} - } - } - Some(data) - } - }; - match ret { - // See the discussion in the stream implementation for why we - // might decrement steals. - Some(data) => unsafe { - if *self.steals.get() > MAX_STEALS { - match self.cnt.swap(0, Ordering::SeqCst) { - DISCONNECTED => { - self.cnt.store(DISCONNECTED, Ordering::SeqCst); - } - n => { - let m = cmp::min(n, *self.steals.get()); - *self.steals.get() -= m; - self.bump(n - m); - } - } - assert!(*self.steals.get() >= 0); - } - *self.steals.get() += 1; - Ok(data) - }, - - // See the discussion in the stream implementation for why we try - // again. - None => { - match self.cnt.load(Ordering::SeqCst) { - n if n != DISCONNECTED => Err(Empty), - _ => { - match self.queue.pop() { - mpsc::Data(t) => Ok(t), - mpsc::Empty => Err(Disconnected), - // with no senders, an inconsistency is impossible. - mpsc::Inconsistent => unreachable!(), - } - } - } - } - } - } - - // Prepares this shared packet for a channel clone, essentially just bumping - // a refcount. - pub fn clone_chan(&self) { - let old_count = self.channels.fetch_add(1, Ordering::SeqCst); - - // See comments on Arc::clone() on why we do this (for `mem::forget`). - if old_count > MAX_REFCOUNT { - abort(); - } - } - - // Decrement the reference count on a channel. This is called whenever a - // Chan is dropped and may end up waking up a receiver. It's the receiver's - // responsibility on the other end to figure out that we've disconnected. - pub fn drop_chan(&self) { - match self.channels.fetch_sub(1, Ordering::SeqCst) { - 1 => {} - n if n > 1 => return, - n => panic!("bad number of channels left {n}"), - } - - match self.cnt.swap(DISCONNECTED, Ordering::SeqCst) { - -1 => { - self.take_to_wake().signal(); - } - DISCONNECTED => {} - n => { - assert!(n >= 0); - } - } - } - - // See the long discussion inside of stream.rs for why the queue is drained, - // and why it is done in this fashion. - pub fn drop_port(&self) { - self.port_dropped.store(true, Ordering::SeqCst); - let mut steals = unsafe { *self.steals.get() }; - while { - match self.cnt.compare_exchange( - steals, - DISCONNECTED, - Ordering::SeqCst, - Ordering::SeqCst, - ) { - Ok(_) => false, - Err(old) => old != DISCONNECTED, - } - } { - // See the discussion in 'try_recv' for why we yield - // control of this thread. - loop { - match self.queue.pop() { - mpsc::Data(..) => { - steals += 1; - } - mpsc::Empty | mpsc::Inconsistent => break, - } - } - } - } - - // Consumes ownership of the 'to_wake' field. - fn take_to_wake(&self) -> SignalToken { - let ptr = self.to_wake.load(Ordering::SeqCst); - self.to_wake.store(EMPTY, Ordering::SeqCst); - assert!(ptr != EMPTY); - unsafe { SignalToken::from_raw(ptr) } - } - - //////////////////////////////////////////////////////////////////////////// - // select implementation - //////////////////////////////////////////////////////////////////////////// - - // increment the count on the channel (used for selection) - fn bump(&self, amt: isize) -> isize { - match self.cnt.fetch_add(amt, Ordering::SeqCst) { - DISCONNECTED => { - self.cnt.store(DISCONNECTED, Ordering::SeqCst); - DISCONNECTED - } - n => n, - } - } - - // Cancels a previous thread waiting on this port, returning whether there's - // data on the port. - // - // This is similar to the stream implementation (hence fewer comments), but - // uses a different value for the "steals" variable. - pub fn abort_selection(&self, _was_upgrade: bool) -> bool { - // Before we do anything else, we bounce on this lock. The reason for - // doing this is to ensure that any upgrade-in-progress is gone and - // done with. Without this bounce, we can race with inherit_blocker - // about looking at and dealing with to_wake. Once we have acquired the - // lock, we are guaranteed that inherit_blocker is done. - { - let _guard = self.select_lock.lock().unwrap(); - } - - // Like the stream implementation, we want to make sure that the count - // on the channel goes non-negative. We don't know how negative the - // stream currently is, so instead of using a steal value of 1, we load - // the channel count and figure out what we should do to make it - // positive. - let steals = { - let cnt = self.cnt.load(Ordering::SeqCst); - if cnt < 0 && cnt != DISCONNECTED { -cnt } else { 0 } - }; - let prev = self.bump(steals + 1); - - if prev == DISCONNECTED { - assert_eq!(self.to_wake.load(Ordering::SeqCst), EMPTY); - true - } else { - let cur = prev + steals + 1; - assert!(cur >= 0); - if prev < 0 { - drop(self.take_to_wake()); - } else { - while self.to_wake.load(Ordering::SeqCst) != EMPTY { - thread::yield_now(); - } - } - unsafe { - // if the number of steals is -1, it was the pre-emptive -1 steal - // count from when we inherited a blocker. This is fine because - // we're just going to overwrite it with a real value. - let old = self.steals.get(); - assert!(*old == 0 || *old == -1); - *old = steals; - prev >= 0 - } - } - } -} - -impl<T> Drop for Packet<T> { - fn drop(&mut self) { - // Note that this load is not only an assert for correctness about - // disconnection, but also a proper fence before the read of - // `to_wake`, so this assert cannot be removed with also removing - // the `to_wake` assert. - assert_eq!(self.cnt.load(Ordering::SeqCst), DISCONNECTED); - assert_eq!(self.to_wake.load(Ordering::SeqCst), EMPTY); - assert_eq!(self.channels.load(Ordering::SeqCst), 0); - } -} diff --git a/library/std/src/sync/mpsc/spsc_queue.rs b/library/std/src/sync/mpsc/spsc_queue.rs deleted file mode 100644 index 61f91313ea9..00000000000 --- a/library/std/src/sync/mpsc/spsc_queue.rs +++ /dev/null @@ -1,244 +0,0 @@ -//! A single-producer single-consumer concurrent queue -//! -//! This module contains the implementation of an SPSC queue which can be used -//! concurrently between two threads. This data structure is safe to use and -//! enforces the semantics that there is one pusher and one popper. - -// The original implementation is based off: -// https://www.1024cores.net/home/lock-free-algorithms/queues/unbounded-spsc-queue -// -// Note that back when the code was imported, it was licensed under the BSD-2-Clause license: -// http://web.archive.org/web/20110411011612/https://www.1024cores.net/home/lock-free-algorithms/queues/unbounded-spsc-queue -// -// The original author of the code agreed to relicense it under `MIT OR Apache-2.0` in 2017, so as -// of today the license of this file is the same as the rest of the codebase: -// https://github.com/rust-lang/rust/pull/42149 - -#[cfg(all(test, not(target_os = "emscripten")))] -mod tests; - -use core::cell::UnsafeCell; -use core::ptr; - -use crate::boxed::Box; -use crate::sync::atomic::{AtomicPtr, AtomicUsize, Ordering}; - -use super::cache_aligned::CacheAligned; - -// Node within the linked list queue of messages to send -struct Node<T> { - // FIXME: this could be an uninitialized T if we're careful enough, and - // that would reduce memory usage (and be a bit faster). - // is it worth it? - value: Option<T>, // nullable for re-use of nodes - cached: bool, // This node goes into the node cache - next: AtomicPtr<Node<T>>, // next node in the queue -} - -/// The single-producer single-consumer queue. This structure is not cloneable, -/// but it can be safely shared in an Arc if it is guaranteed that there -/// is only one popper and one pusher touching the queue at any one point in -/// time. -pub struct Queue<T, ProducerAddition = (), ConsumerAddition = ()> { - // consumer fields - consumer: CacheAligned<Consumer<T, ConsumerAddition>>, - - // producer fields - producer: CacheAligned<Producer<T, ProducerAddition>>, -} - -struct Consumer<T, Addition> { - tail: UnsafeCell<*mut Node<T>>, // where to pop from - tail_prev: AtomicPtr<Node<T>>, // where to pop from - cache_bound: usize, // maximum cache size - cached_nodes: AtomicUsize, // number of nodes marked as cacheable - addition: Addition, -} - -struct Producer<T, Addition> { - head: UnsafeCell<*mut Node<T>>, // where to push to - first: UnsafeCell<*mut Node<T>>, // where to get new nodes from - tail_copy: UnsafeCell<*mut Node<T>>, // between first/tail - addition: Addition, -} - -unsafe impl<T: Send, P: Send + Sync, C: Send + Sync> Send for Queue<T, P, C> {} - -unsafe impl<T: Send, P: Send + Sync, C: Send + Sync> Sync for Queue<T, P, C> {} - -impl<T> Node<T> { - fn new() -> *mut Node<T> { - Box::into_raw(box Node { - value: None, - cached: false, - next: AtomicPtr::new(ptr::null_mut::<Node<T>>()), - }) - } -} - -impl<T, ProducerAddition, ConsumerAddition> Queue<T, ProducerAddition, ConsumerAddition> { - /// Creates a new queue. With given additional elements in the producer and - /// consumer portions of the queue. - /// - /// Due to the performance implications of cache-contention, - /// we wish to keep fields used mainly by the producer on a separate cache - /// line than those used by the consumer. - /// Since cache lines are usually 64 bytes, it is unreasonably expensive to - /// allocate one for small fields, so we allow users to insert additional - /// fields into the cache lines already allocated by this for the producer - /// and consumer. - /// - /// This is unsafe as the type system doesn't enforce a single - /// consumer-producer relationship. It also allows the consumer to `pop` - /// items while there is a `peek` active due to all methods having a - /// non-mutable receiver. - /// - /// # Arguments - /// - /// * `bound` - This queue implementation is implemented with a linked - /// list, and this means that a push is always a malloc. In - /// order to amortize this cost, an internal cache of nodes is - /// maintained to prevent a malloc from always being - /// necessary. This bound is the limit on the size of the - /// cache (if desired). If the value is 0, then the cache has - /// no bound. Otherwise, the cache will never grow larger than - /// `bound` (although the queue itself could be much larger. - pub unsafe fn with_additions( - bound: usize, - producer_addition: ProducerAddition, - consumer_addition: ConsumerAddition, - ) -> Self { - let n1 = Node::new(); - let n2 = Node::new(); - (*n1).next.store(n2, Ordering::Relaxed); - Queue { - consumer: CacheAligned::new(Consumer { - tail: UnsafeCell::new(n2), - tail_prev: AtomicPtr::new(n1), - cache_bound: bound, - cached_nodes: AtomicUsize::new(0), - addition: consumer_addition, - }), - producer: CacheAligned::new(Producer { - head: UnsafeCell::new(n2), - first: UnsafeCell::new(n1), - tail_copy: UnsafeCell::new(n1), - addition: producer_addition, - }), - } - } - - /// Pushes a new value onto this queue. Note that to use this function - /// safely, it must be externally guaranteed that there is only one pusher. - pub fn push(&self, t: T) { - unsafe { - // Acquire a node (which either uses a cached one or allocates a new - // one), and then append this to the 'head' node. - let n = self.alloc(); - assert!((*n).value.is_none()); - (*n).value = Some(t); - (*n).next.store(ptr::null_mut(), Ordering::Relaxed); - (**self.producer.head.get()).next.store(n, Ordering::Release); - *(&self.producer.head).get() = n; - } - } - - unsafe fn alloc(&self) -> *mut Node<T> { - // First try to see if we can consume the 'first' node for our uses. - if *self.producer.first.get() != *self.producer.tail_copy.get() { - let ret = *self.producer.first.get(); - *self.producer.0.first.get() = (*ret).next.load(Ordering::Relaxed); - return ret; - } - // If the above fails, then update our copy of the tail and try - // again. - *self.producer.0.tail_copy.get() = self.consumer.tail_prev.load(Ordering::Acquire); - if *self.producer.first.get() != *self.producer.tail_copy.get() { - let ret = *self.producer.first.get(); - *self.producer.0.first.get() = (*ret).next.load(Ordering::Relaxed); - return ret; - } - // If all of that fails, then we have to allocate a new node - // (there's nothing in the node cache). - Node::new() - } - - /// Attempts to pop a value from this queue. Remember that to use this type - /// safely you must ensure that there is only one popper at a time. - pub fn pop(&self) -> Option<T> { - unsafe { - // The `tail` node is not actually a used node, but rather a - // sentinel from where we should start popping from. Hence, look at - // tail's next field and see if we can use it. If we do a pop, then - // the current tail node is a candidate for going into the cache. - let tail = *self.consumer.tail.get(); - let next = (*tail).next.load(Ordering::Acquire); - if next.is_null() { - return None; - } - assert!((*next).value.is_some()); - let ret = (*next).value.take(); - - *self.consumer.0.tail.get() = next; - if self.consumer.cache_bound == 0 { - self.consumer.tail_prev.store(tail, Ordering::Release); - } else { - let cached_nodes = self.consumer.cached_nodes.load(Ordering::Relaxed); - if cached_nodes < self.consumer.cache_bound && !(*tail).cached { - self.consumer.cached_nodes.store(cached_nodes, Ordering::Relaxed); - (*tail).cached = true; - } - - if (*tail).cached { - self.consumer.tail_prev.store(tail, Ordering::Release); - } else { - (*self.consumer.tail_prev.load(Ordering::Relaxed)) - .next - .store(next, Ordering::Relaxed); - // We have successfully erased all references to 'tail', so - // now we can safely drop it. - let _: Box<Node<T>> = Box::from_raw(tail); - } - } - ret - } - } - - /// Attempts to peek at the head of the queue, returning `None` if the queue - /// has no data currently - /// - /// # Warning - /// The reference returned is invalid if it is not used before the consumer - /// pops the value off the queue. If the producer then pushes another value - /// onto the queue, it will overwrite the value pointed to by the reference. - pub fn peek(&self) -> Option<&mut T> { - // This is essentially the same as above with all the popping bits - // stripped out. - unsafe { - let tail = *self.consumer.tail.get(); - let next = (*tail).next.load(Ordering::Acquire); - if next.is_null() { None } else { (*next).value.as_mut() } - } - } - - pub fn producer_addition(&self) -> &ProducerAddition { - &self.producer.addition - } - - pub fn consumer_addition(&self) -> &ConsumerAddition { - &self.consumer.addition - } -} - -impl<T, ProducerAddition, ConsumerAddition> Drop for Queue<T, ProducerAddition, ConsumerAddition> { - fn drop(&mut self) { - unsafe { - let mut cur = *self.producer.first.get(); - while !cur.is_null() { - let next = (*cur).next.load(Ordering::Relaxed); - let _n: Box<Node<T>> = Box::from_raw(cur); - cur = next; - } - } - } -} diff --git a/library/std/src/sync/mpsc/spsc_queue/tests.rs b/library/std/src/sync/mpsc/spsc_queue/tests.rs deleted file mode 100644 index eb6d5c2cf66..00000000000 --- a/library/std/src/sync/mpsc/spsc_queue/tests.rs +++ /dev/null @@ -1,102 +0,0 @@ -use super::Queue; -use crate::sync::mpsc::channel; -use crate::sync::Arc; -use crate::thread; - -#[test] -fn smoke() { - unsafe { - let queue = Queue::with_additions(0, (), ()); - queue.push(1); - queue.push(2); - assert_eq!(queue.pop(), Some(1)); - assert_eq!(queue.pop(), Some(2)); - assert_eq!(queue.pop(), None); - queue.push(3); - queue.push(4); - assert_eq!(queue.pop(), Some(3)); - assert_eq!(queue.pop(), Some(4)); - assert_eq!(queue.pop(), None); - } -} - -#[test] -fn peek() { - unsafe { - let queue = Queue::with_additions(0, (), ()); - queue.push(vec![1]); - - // Ensure the borrowchecker works - match queue.peek() { - Some(vec) => { - assert_eq!(&*vec, &[1]); - } - None => unreachable!(), - } - - match queue.pop() { - Some(vec) => { - assert_eq!(&*vec, &[1]); - } - None => unreachable!(), - } - } -} - -#[test] -fn drop_full() { - unsafe { - let q: Queue<Box<_>> = Queue::with_additions(0, (), ()); - q.push(Box::new(1)); - q.push(Box::new(2)); - } -} - -#[test] -fn smoke_bound() { - unsafe { - let q = Queue::with_additions(0, (), ()); - q.push(1); - q.push(2); - assert_eq!(q.pop(), Some(1)); - assert_eq!(q.pop(), Some(2)); - assert_eq!(q.pop(), None); - q.push(3); - q.push(4); - assert_eq!(q.pop(), Some(3)); - assert_eq!(q.pop(), Some(4)); - assert_eq!(q.pop(), None); - } -} - -#[test] -fn stress() { - unsafe { - stress_bound(0); - stress_bound(1); - } - - unsafe fn stress_bound(bound: usize) { - let count = if cfg!(miri) { 1000 } else { 100000 }; - let q = Arc::new(Queue::with_additions(bound, (), ())); - - let (tx, rx) = channel(); - let q2 = q.clone(); - let _t = thread::spawn(move || { - for _ in 0..count { - loop { - match q2.pop() { - Some(1) => break, - Some(_) => panic!(), - None => {} - } - } - } - tx.send(()).unwrap(); - }); - for _ in 0..count { - q.push(1); - } - rx.recv().unwrap(); - } -} diff --git a/library/std/src/sync/mpsc/stream.rs b/library/std/src/sync/mpsc/stream.rs deleted file mode 100644 index 4592e914160..00000000000 --- a/library/std/src/sync/mpsc/stream.rs +++ /dev/null @@ -1,457 +0,0 @@ -/// Stream channels -/// -/// This is the flavor of channels which are optimized for one sender and one -/// receiver. The sender will be upgraded to a shared channel if the channel is -/// cloned. -/// -/// High level implementation details can be found in the comment of the parent -/// module. -pub use self::Failure::*; -use self::Message::*; -pub use self::UpgradeResult::*; - -use core::cmp; - -use crate::cell::UnsafeCell; -use crate::ptr; -use crate::thread; -use crate::time::Instant; - -use crate::sync::atomic::{AtomicBool, AtomicIsize, AtomicPtr, Ordering}; -use crate::sync::mpsc::blocking::{self, SignalToken}; -use crate::sync::mpsc::spsc_queue as spsc; -use crate::sync::mpsc::Receiver; - -const DISCONNECTED: isize = isize::MIN; -#[cfg(test)] -const MAX_STEALS: isize = 5; -#[cfg(not(test))] -const MAX_STEALS: isize = 1 << 20; -const EMPTY: *mut u8 = ptr::null_mut(); // initial state: no data, no blocked receiver - -pub struct Packet<T> { - // internal queue for all messages - queue: spsc::Queue<Message<T>, ProducerAddition, ConsumerAddition>, -} - -struct ProducerAddition { - cnt: AtomicIsize, // How many items are on this channel - to_wake: AtomicPtr<u8>, // SignalToken for the blocked thread to wake up - - port_dropped: AtomicBool, // flag if the channel has been destroyed. -} - -struct ConsumerAddition { - steals: UnsafeCell<isize>, // How many times has a port received without blocking? -} - -pub enum Failure<T> { - Empty, - Disconnected, - Upgraded(Receiver<T>), -} - -pub enum UpgradeResult { - UpSuccess, - UpDisconnected, - UpWoke(SignalToken), -} - -// Any message could contain an "upgrade request" to a new shared port, so the -// internal queue it's a queue of T, but rather Message<T> -enum Message<T> { - Data(T), - GoUp(Receiver<T>), -} - -impl<T> Packet<T> { - pub fn new() -> Packet<T> { - Packet { - queue: unsafe { - spsc::Queue::with_additions( - 128, - ProducerAddition { - cnt: AtomicIsize::new(0), - to_wake: AtomicPtr::new(EMPTY), - - port_dropped: AtomicBool::new(false), - }, - ConsumerAddition { steals: UnsafeCell::new(0) }, - ) - }, - } - } - - pub fn send(&self, t: T) -> Result<(), T> { - // If the other port has deterministically gone away, then definitely - // must return the data back up the stack. Otherwise, the data is - // considered as being sent. - if self.queue.producer_addition().port_dropped.load(Ordering::SeqCst) { - return Err(t); - } - - match self.do_send(Data(t)) { - UpSuccess | UpDisconnected => {} - UpWoke(token) => { - token.signal(); - } - } - Ok(()) - } - - pub fn upgrade(&self, up: Receiver<T>) -> UpgradeResult { - // If the port has gone away, then there's no need to proceed any - // further. - if self.queue.producer_addition().port_dropped.load(Ordering::SeqCst) { - return UpDisconnected; - } - - self.do_send(GoUp(up)) - } - - fn do_send(&self, t: Message<T>) -> UpgradeResult { - self.queue.push(t); - match self.queue.producer_addition().cnt.fetch_add(1, Ordering::SeqCst) { - // As described in the mod's doc comment, -1 == wakeup - -1 => UpWoke(self.take_to_wake()), - // As described before, SPSC queues must be >= -2 - -2 => UpSuccess, - - // Be sure to preserve the disconnected state, and the return value - // in this case is going to be whether our data was received or not. - // This manifests itself on whether we have an empty queue or not. - // - // Primarily, are required to drain the queue here because the port - // will never remove this data. We can only have at most one item to - // drain (the port drains the rest). - DISCONNECTED => { - self.queue.producer_addition().cnt.store(DISCONNECTED, Ordering::SeqCst); - let first = self.queue.pop(); - let second = self.queue.pop(); - assert!(second.is_none()); - - match first { - Some(..) => UpSuccess, // we failed to send the data - None => UpDisconnected, // we successfully sent data - } - } - - // Otherwise we just sent some data on a non-waiting queue, so just - // make sure the world is sane and carry on! - n => { - assert!(n >= 0); - UpSuccess - } - } - } - - // Consumes ownership of the 'to_wake' field. - fn take_to_wake(&self) -> SignalToken { - let ptr = self.queue.producer_addition().to_wake.load(Ordering::SeqCst); - self.queue.producer_addition().to_wake.store(EMPTY, Ordering::SeqCst); - assert!(ptr != EMPTY); - unsafe { SignalToken::from_raw(ptr) } - } - - // Decrements the count on the channel for a sleeper, returning the sleeper - // back if it shouldn't sleep. Note that this is the location where we take - // steals into account. - fn decrement(&self, token: SignalToken) -> Result<(), SignalToken> { - assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), EMPTY); - let ptr = unsafe { token.to_raw() }; - self.queue.producer_addition().to_wake.store(ptr, Ordering::SeqCst); - - let steals = unsafe { ptr::replace(self.queue.consumer_addition().steals.get(), 0) }; - - match self.queue.producer_addition().cnt.fetch_sub(1 + steals, Ordering::SeqCst) { - DISCONNECTED => { - self.queue.producer_addition().cnt.store(DISCONNECTED, Ordering::SeqCst); - } - // If we factor in our steals and notice that the channel has no - // data, we successfully sleep - n => { - assert!(n >= 0); - if n - steals <= 0 { - return Ok(()); - } - } - } - - self.queue.producer_addition().to_wake.store(EMPTY, Ordering::SeqCst); - Err(unsafe { SignalToken::from_raw(ptr) }) - } - - pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure<T>> { - // Optimistic preflight check (scheduling is expensive). - match self.try_recv() { - Err(Empty) => {} - data => return data, - } - - // Welp, our channel has no data. Deschedule the current thread and - // initiate the blocking protocol. - let (wait_token, signal_token) = blocking::tokens(); - if self.decrement(signal_token).is_ok() { - if let Some(deadline) = deadline { - let timed_out = !wait_token.wait_max_until(deadline); - if timed_out { - self.abort_selection(/* was_upgrade = */ false).map_err(Upgraded)?; - } - } else { - wait_token.wait(); - } - } - - match self.try_recv() { - // Messages which actually popped from the queue shouldn't count as - // a steal, so offset the decrement here (we already have our - // "steal" factored into the channel count above). - data @ (Ok(..) | Err(Upgraded(..))) => unsafe { - *self.queue.consumer_addition().steals.get() -= 1; - data - }, - - data => data, - } - } - - pub fn try_recv(&self) -> Result<T, Failure<T>> { - match self.queue.pop() { - // If we stole some data, record to that effect (this will be - // factored into cnt later on). - // - // Note that we don't allow steals to grow without bound in order to - // prevent eventual overflow of either steals or cnt as an overflow - // would have catastrophic results. Sometimes, steals > cnt, but - // other times cnt > steals, so we don't know the relation between - // steals and cnt. This code path is executed only rarely, so we do - // a pretty slow operation, of swapping 0 into cnt, taking steals - // down as much as possible (without going negative), and then - // adding back in whatever we couldn't factor into steals. - Some(data) => unsafe { - if *self.queue.consumer_addition().steals.get() > MAX_STEALS { - match self.queue.producer_addition().cnt.swap(0, Ordering::SeqCst) { - DISCONNECTED => { - self.queue - .producer_addition() - .cnt - .store(DISCONNECTED, Ordering::SeqCst); - } - n => { - let m = cmp::min(n, *self.queue.consumer_addition().steals.get()); - *self.queue.consumer_addition().steals.get() -= m; - self.bump(n - m); - } - } - assert!(*self.queue.consumer_addition().steals.get() >= 0); - } - *self.queue.consumer_addition().steals.get() += 1; - match data { - Data(t) => Ok(t), - GoUp(up) => Err(Upgraded(up)), - } - }, - - None => { - match self.queue.producer_addition().cnt.load(Ordering::SeqCst) { - n if n != DISCONNECTED => Err(Empty), - - // This is a little bit of a tricky case. We failed to pop - // data above, and then we have viewed that the channel is - // disconnected. In this window more data could have been - // sent on the channel. It doesn't really make sense to - // return that the channel is disconnected when there's - // actually data on it, so be extra sure there's no data by - // popping one more time. - // - // We can ignore steals because the other end is - // disconnected and we'll never need to really factor in our - // steals again. - _ => match self.queue.pop() { - Some(Data(t)) => Ok(t), - Some(GoUp(up)) => Err(Upgraded(up)), - None => Err(Disconnected), - }, - } - } - } - } - - pub fn drop_chan(&self) { - // Dropping a channel is pretty simple, we just flag it as disconnected - // and then wakeup a blocker if there is one. - match self.queue.producer_addition().cnt.swap(DISCONNECTED, Ordering::SeqCst) { - -1 => { - self.take_to_wake().signal(); - } - DISCONNECTED => {} - n => { - assert!(n >= 0); - } - } - } - - pub fn drop_port(&self) { - // Dropping a port seems like a fairly trivial thing. In theory all we - // need to do is flag that we're disconnected and then everything else - // can take over (we don't have anyone to wake up). - // - // The catch for Ports is that we want to drop the entire contents of - // the queue. There are multiple reasons for having this property, the - // largest of which is that if another chan is waiting in this channel - // (but not received yet), then waiting on that port will cause a - // deadlock. - // - // So if we accept that we must now destroy the entire contents of the - // queue, this code may make a bit more sense. The tricky part is that - // we can't let any in-flight sends go un-dropped, we have to make sure - // *everything* is dropped and nothing new will come onto the channel. - - // The first thing we do is set a flag saying that we're done for. All - // sends are gated on this flag, so we're immediately guaranteed that - // there are a bounded number of active sends that we'll have to deal - // with. - self.queue.producer_addition().port_dropped.store(true, Ordering::SeqCst); - - // Now that we're guaranteed to deal with a bounded number of senders, - // we need to drain the queue. This draining process happens atomically - // with respect to the "count" of the channel. If the count is nonzero - // (with steals taken into account), then there must be data on the - // channel. In this case we drain everything and then try again. We will - // continue to fail while active senders send data while we're dropping - // data, but eventually we're guaranteed to break out of this loop - // (because there is a bounded number of senders). - let mut steals = unsafe { *self.queue.consumer_addition().steals.get() }; - while { - match self.queue.producer_addition().cnt.compare_exchange( - steals, - DISCONNECTED, - Ordering::SeqCst, - Ordering::SeqCst, - ) { - Ok(_) => false, - Err(old) => old != DISCONNECTED, - } - } { - while self.queue.pop().is_some() { - steals += 1; - } - } - - // At this point in time, we have gated all future senders from sending, - // and we have flagged the channel as being disconnected. The senders - // still have some responsibility, however, because some sends might not - // complete until after we flag the disconnection. There are more - // details in the sending methods that see DISCONNECTED - } - - //////////////////////////////////////////////////////////////////////////// - // select implementation - //////////////////////////////////////////////////////////////////////////// - - // increment the count on the channel (used for selection) - fn bump(&self, amt: isize) -> isize { - match self.queue.producer_addition().cnt.fetch_add(amt, Ordering::SeqCst) { - DISCONNECTED => { - self.queue.producer_addition().cnt.store(DISCONNECTED, Ordering::SeqCst); - DISCONNECTED - } - n => n, - } - } - - // Removes a previous thread from being blocked in this port - pub fn abort_selection(&self, was_upgrade: bool) -> Result<bool, Receiver<T>> { - // If we're aborting selection after upgrading from a oneshot, then - // we're guarantee that no one is waiting. The only way that we could - // have seen the upgrade is if data was actually sent on the channel - // half again. For us, this means that there is guaranteed to be data on - // this channel. Furthermore, we're guaranteed that there was no - // start_selection previously, so there's no need to modify `self.cnt` - // at all. - // - // Hence, because of these invariants, we immediately return `Ok(true)`. - // Note that the data might not actually be sent on the channel just yet. - // The other end could have flagged the upgrade but not sent data to - // this end. This is fine because we know it's a small bounded windows - // of time until the data is actually sent. - if was_upgrade { - assert_eq!(unsafe { *self.queue.consumer_addition().steals.get() }, 0); - assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), EMPTY); - return Ok(true); - } - - // We want to make sure that the count on the channel goes non-negative, - // and in the stream case we can have at most one steal, so just assume - // that we had one steal. - let steals = 1; - let prev = self.bump(steals + 1); - - // If we were previously disconnected, then we know for sure that there - // is no thread in to_wake, so just keep going - let has_data = if prev == DISCONNECTED { - assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), EMPTY); - true // there is data, that data is that we're disconnected - } else { - let cur = prev + steals + 1; - assert!(cur >= 0); - - // If the previous count was negative, then we just made things go - // positive, hence we passed the -1 boundary and we're responsible - // for removing the to_wake() field and trashing it. - // - // If the previous count was positive then we're in a tougher - // situation. A possible race is that a sender just incremented - // through -1 (meaning it's going to try to wake a thread up), but it - // hasn't yet read the to_wake. In order to prevent a future recv() - // from waking up too early (this sender picking up the plastered - // over to_wake), we spin loop here waiting for to_wake to be 0. - // Note that this entire select() implementation needs an overhaul, - // and this is *not* the worst part of it, so this is not done as a - // final solution but rather out of necessity for now to get - // something working. - if prev < 0 { - drop(self.take_to_wake()); - } else { - while self.queue.producer_addition().to_wake.load(Ordering::SeqCst) != EMPTY { - thread::yield_now(); - } - } - unsafe { - assert_eq!(*self.queue.consumer_addition().steals.get(), 0); - *self.queue.consumer_addition().steals.get() = steals; - } - - // if we were previously positive, then there's surely data to - // receive - prev >= 0 - }; - - // Now that we've determined that this queue "has data", we peek at the - // queue to see if the data is an upgrade or not. If it's an upgrade, - // then we need to destroy this port and abort selection on the - // upgraded port. - if has_data { - match self.queue.peek() { - Some(&mut GoUp(..)) => match self.queue.pop() { - Some(GoUp(port)) => Err(port), - _ => unreachable!(), - }, - _ => Ok(true), - } - } else { - Ok(false) - } - } -} - -impl<T> Drop for Packet<T> { - fn drop(&mut self) { - // Note that this load is not only an assert for correctness about - // disconnection, but also a proper fence before the read of - // `to_wake`, so this assert cannot be removed with also removing - // the `to_wake` assert. - assert_eq!(self.queue.producer_addition().cnt.load(Ordering::SeqCst), DISCONNECTED); - assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), EMPTY); - } -} diff --git a/library/std/src/sync/mpsc/sync.rs b/library/std/src/sync/mpsc/sync.rs deleted file mode 100644 index 733761671a0..00000000000 --- a/library/std/src/sync/mpsc/sync.rs +++ /dev/null @@ -1,495 +0,0 @@ -use self::Blocker::*; -/// Synchronous channels/ports -/// -/// This channel implementation differs significantly from the asynchronous -/// implementations found next to it (oneshot/stream/share). This is an -/// implementation of a synchronous, bounded buffer channel. -/// -/// Each channel is created with some amount of backing buffer, and sends will -/// *block* until buffer space becomes available. A buffer size of 0 is valid, -/// which means that every successful send is paired with a successful recv. -/// -/// This flavor of channels defines a new `send_opt` method for channels which -/// is the method by which a message is sent but the thread does not panic if it -/// cannot be delivered. -/// -/// Another major difference is that send() will *always* return back the data -/// if it couldn't be sent. This is because it is deterministically known when -/// the data is received and when it is not received. -/// -/// Implementation-wise, it can all be summed up with "use a mutex plus some -/// logic". The mutex used here is an OS native mutex, meaning that no user code -/// is run inside of the mutex (to prevent context switching). This -/// implementation shares almost all code for the buffered and unbuffered cases -/// of a synchronous channel. There are a few branches for the unbuffered case, -/// but they're mostly just relevant to blocking senders. -pub use self::Failure::*; - -use core::intrinsics::abort; -use core::mem; -use core::ptr; - -use crate::sync::atomic::{AtomicUsize, Ordering}; -use crate::sync::mpsc::blocking::{self, SignalToken, WaitToken}; -use crate::sync::{Mutex, MutexGuard}; -use crate::time::Instant; - -const MAX_REFCOUNT: usize = (isize::MAX) as usize; - -pub struct Packet<T> { - /// Only field outside of the mutex. Just done for kicks, but mainly because - /// the other shared channel already had the code implemented - channels: AtomicUsize, - - lock: Mutex<State<T>>, -} - -unsafe impl<T: Send> Send for Packet<T> {} - -unsafe impl<T: Send> Sync for Packet<T> {} - -struct State<T> { - disconnected: bool, // Is the channel disconnected yet? - queue: Queue, // queue of senders waiting to send data - blocker: Blocker, // currently blocked thread on this channel - buf: Buffer<T>, // storage for buffered messages - cap: usize, // capacity of this channel - - /// A curious flag used to indicate whether a sender failed or succeeded in - /// blocking. This is used to transmit information back to the thread that it - /// must dequeue its message from the buffer because it was not received. - /// This is only relevant in the 0-buffer case. This obviously cannot be - /// safely constructed, but it's guaranteed to always have a valid pointer - /// value. - canceled: Option<&'static mut bool>, -} - -unsafe impl<T: Send> Send for State<T> {} - -/// Possible flavors of threads who can be blocked on this channel. -enum Blocker { - BlockedSender(SignalToken), - BlockedReceiver(SignalToken), - NoneBlocked, -} - -/// Simple queue for threading threads together. Nodes are stack-allocated, so -/// this structure is not safe at all -struct Queue { - head: *mut Node, - tail: *mut Node, -} - -struct Node { - token: Option<SignalToken>, - next: *mut Node, -} - -unsafe impl Send for Node {} - -/// A simple ring-buffer -struct Buffer<T> { - buf: Vec<Option<T>>, - start: usize, - size: usize, -} - -#[derive(Debug)] -pub enum Failure { - Empty, - Disconnected, -} - -/// Atomically blocks the current thread, placing it into `slot`, unlocking `lock` -/// in the meantime. This re-locks the mutex upon returning. -fn wait<'a, 'b, T>( - lock: &'a Mutex<State<T>>, - mut guard: MutexGuard<'b, State<T>>, - f: fn(SignalToken) -> Blocker, -) -> MutexGuard<'a, State<T>> { - let (wait_token, signal_token) = blocking::tokens(); - match mem::replace(&mut guard.blocker, f(signal_token)) { - NoneBlocked => {} - _ => unreachable!(), - } - drop(guard); // unlock - wait_token.wait(); // block - lock.lock().unwrap() // relock -} - -/// Same as wait, but waiting at most until `deadline`. -fn wait_timeout_receiver<'a, 'b, T>( - lock: &'a Mutex<State<T>>, - deadline: Instant, - mut guard: MutexGuard<'b, State<T>>, - success: &mut bool, -) -> MutexGuard<'a, State<T>> { - let (wait_token, signal_token) = blocking::tokens(); - match mem::replace(&mut guard.blocker, BlockedReceiver(signal_token)) { - NoneBlocked => {} - _ => unreachable!(), - } - drop(guard); // unlock - *success = wait_token.wait_max_until(deadline); // block - let mut new_guard = lock.lock().unwrap(); // relock - if !*success { - abort_selection(&mut new_guard); - } - new_guard -} - -fn abort_selection<T>(guard: &mut MutexGuard<'_, State<T>>) -> bool { - match mem::replace(&mut guard.blocker, NoneBlocked) { - NoneBlocked => true, - BlockedSender(token) => { - guard.blocker = BlockedSender(token); - true - } - BlockedReceiver(token) => { - drop(token); - false - } - } -} - -/// Wakes up a thread, dropping the lock at the correct time -fn wakeup<T>(token: SignalToken, guard: MutexGuard<'_, State<T>>) { - // We need to be careful to wake up the waiting thread *outside* of the mutex - // in case it incurs a context switch. - drop(guard); - token.signal(); -} - -impl<T> Packet<T> { - pub fn new(capacity: usize) -> Packet<T> { - Packet { - channels: AtomicUsize::new(1), - lock: Mutex::new(State { - disconnected: false, - blocker: NoneBlocked, - cap: capacity, - canceled: None, - queue: Queue { head: ptr::null_mut(), tail: ptr::null_mut() }, - buf: Buffer { - buf: (0..capacity + if capacity == 0 { 1 } else { 0 }).map(|_| None).collect(), - start: 0, - size: 0, - }, - }), - } - } - - // wait until a send slot is available, returning locked access to - // the channel state. - fn acquire_send_slot(&self) -> MutexGuard<'_, State<T>> { - let mut node = Node { token: None, next: ptr::null_mut() }; - loop { - let mut guard = self.lock.lock().unwrap(); - // are we ready to go? - if guard.disconnected || guard.buf.size() < guard.buf.capacity() { - return guard; - } - // no room; actually block - let wait_token = guard.queue.enqueue(&mut node); - drop(guard); - wait_token.wait(); - } - } - - pub fn send(&self, t: T) -> Result<(), T> { - let mut guard = self.acquire_send_slot(); - if guard.disconnected { - return Err(t); - } - guard.buf.enqueue(t); - - match mem::replace(&mut guard.blocker, NoneBlocked) { - // if our capacity is 0, then we need to wait for a receiver to be - // available to take our data. After waiting, we check again to make - // sure the port didn't go away in the meantime. If it did, we need - // to hand back our data. - NoneBlocked if guard.cap == 0 => { - let mut canceled = false; - assert!(guard.canceled.is_none()); - guard.canceled = Some(unsafe { mem::transmute(&mut canceled) }); - let mut guard = wait(&self.lock, guard, BlockedSender); - if canceled { Err(guard.buf.dequeue()) } else { Ok(()) } - } - - // success, we buffered some data - NoneBlocked => Ok(()), - - // success, someone's about to receive our buffered data. - BlockedReceiver(token) => { - wakeup(token, guard); - Ok(()) - } - - BlockedSender(..) => panic!("lolwut"), - } - } - - pub fn try_send(&self, t: T) -> Result<(), super::TrySendError<T>> { - let mut guard = self.lock.lock().unwrap(); - if guard.disconnected { - Err(super::TrySendError::Disconnected(t)) - } else if guard.buf.size() == guard.buf.capacity() { - Err(super::TrySendError::Full(t)) - } else if guard.cap == 0 { - // With capacity 0, even though we have buffer space we can't - // transfer the data unless there's a receiver waiting. - match mem::replace(&mut guard.blocker, NoneBlocked) { - NoneBlocked => Err(super::TrySendError::Full(t)), - BlockedSender(..) => unreachable!(), - BlockedReceiver(token) => { - guard.buf.enqueue(t); - wakeup(token, guard); - Ok(()) - } - } - } else { - // If the buffer has some space and the capacity isn't 0, then we - // just enqueue the data for later retrieval, ensuring to wake up - // any blocked receiver if there is one. - assert!(guard.buf.size() < guard.buf.capacity()); - guard.buf.enqueue(t); - match mem::replace(&mut guard.blocker, NoneBlocked) { - BlockedReceiver(token) => wakeup(token, guard), - NoneBlocked => {} - BlockedSender(..) => unreachable!(), - } - Ok(()) - } - } - - // Receives a message from this channel - // - // When reading this, remember that there can only ever be one receiver at - // time. - pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure> { - let mut guard = self.lock.lock().unwrap(); - - let mut woke_up_after_waiting = false; - // Wait for the buffer to have something in it. No need for a - // while loop because we're the only receiver. - if !guard.disconnected && guard.buf.size() == 0 { - if let Some(deadline) = deadline { - guard = - wait_timeout_receiver(&self.lock, deadline, guard, &mut woke_up_after_waiting); - } else { - guard = wait(&self.lock, guard, BlockedReceiver); - woke_up_after_waiting = true; - } - } - - // N.B., channel could be disconnected while waiting, so the order of - // these conditionals is important. - if guard.disconnected && guard.buf.size() == 0 { - return Err(Disconnected); - } - - // Pick up the data, wake up our neighbors, and carry on - assert!(guard.buf.size() > 0 || (deadline.is_some() && !woke_up_after_waiting)); - - if guard.buf.size() == 0 { - return Err(Empty); - } - - let ret = guard.buf.dequeue(); - self.wakeup_senders(woke_up_after_waiting, guard); - Ok(ret) - } - - pub fn try_recv(&self) -> Result<T, Failure> { - let mut guard = self.lock.lock().unwrap(); - - // Easy cases first - if guard.disconnected && guard.buf.size() == 0 { - return Err(Disconnected); - } - if guard.buf.size() == 0 { - return Err(Empty); - } - - // Be sure to wake up neighbors - let ret = Ok(guard.buf.dequeue()); - self.wakeup_senders(false, guard); - ret - } - - // Wake up pending senders after some data has been received - // - // * `waited` - flag if the receiver blocked to receive some data, or if it - // just picked up some data on the way out - // * `guard` - the lock guard that is held over this channel's lock - fn wakeup_senders(&self, waited: bool, mut guard: MutexGuard<'_, State<T>>) { - let pending_sender1: Option<SignalToken> = guard.queue.dequeue(); - - // If this is a no-buffer channel (cap == 0), then if we didn't wait we - // need to ACK the sender. If we waited, then the sender waking us up - // was already the ACK. - let pending_sender2 = if guard.cap == 0 && !waited { - match mem::replace(&mut guard.blocker, NoneBlocked) { - NoneBlocked => None, - BlockedReceiver(..) => unreachable!(), - BlockedSender(token) => { - guard.canceled.take(); - Some(token) - } - } - } else { - None - }; - mem::drop(guard); - - // only outside of the lock do we wake up the pending threads - if let Some(token) = pending_sender1 { - token.signal(); - } - if let Some(token) = pending_sender2 { - token.signal(); - } - } - - // Prepares this shared packet for a channel clone, essentially just bumping - // a refcount. - pub fn clone_chan(&self) { - let old_count = self.channels.fetch_add(1, Ordering::SeqCst); - - // See comments on Arc::clone() on why we do this (for `mem::forget`). - if old_count > MAX_REFCOUNT { - abort(); - } - } - - pub fn drop_chan(&self) { - // Only flag the channel as disconnected if we're the last channel - match self.channels.fetch_sub(1, Ordering::SeqCst) { - 1 => {} - _ => return, - } - - // Not much to do other than wake up a receiver if one's there - let mut guard = self.lock.lock().unwrap(); - if guard.disconnected { - return; - } - guard.disconnected = true; - match mem::replace(&mut guard.blocker, NoneBlocked) { - NoneBlocked => {} - BlockedSender(..) => unreachable!(), - BlockedReceiver(token) => wakeup(token, guard), - } - } - - pub fn drop_port(&self) { - let mut guard = self.lock.lock().unwrap(); - - if guard.disconnected { - return; - } - guard.disconnected = true; - - // If the capacity is 0, then the sender may want its data back after - // we're disconnected. Otherwise it's now our responsibility to destroy - // the buffered data. As with many other portions of this code, this - // needs to be careful to destroy the data *outside* of the lock to - // prevent deadlock. - let _data = if guard.cap != 0 { mem::take(&mut guard.buf.buf) } else { Vec::new() }; - let mut queue = - mem::replace(&mut guard.queue, Queue { head: ptr::null_mut(), tail: ptr::null_mut() }); - - let waiter = match mem::replace(&mut guard.blocker, NoneBlocked) { - NoneBlocked => None, - BlockedSender(token) => { - *guard.canceled.take().unwrap() = true; - Some(token) - } - BlockedReceiver(..) => unreachable!(), - }; - mem::drop(guard); - - while let Some(token) = queue.dequeue() { - token.signal(); - } - if let Some(token) = waiter { - token.signal(); - } - } -} - -impl<T> Drop for Packet<T> { - fn drop(&mut self) { - assert_eq!(self.channels.load(Ordering::SeqCst), 0); - let mut guard = self.lock.lock().unwrap(); - assert!(guard.queue.dequeue().is_none()); - assert!(guard.canceled.is_none()); - } -} - -//////////////////////////////////////////////////////////////////////////////// -// Buffer, a simple ring buffer backed by Vec<T> -//////////////////////////////////////////////////////////////////////////////// - -impl<T> Buffer<T> { - fn enqueue(&mut self, t: T) { - let pos = (self.start + self.size) % self.buf.len(); - self.size += 1; - let prev = mem::replace(&mut self.buf[pos], Some(t)); - assert!(prev.is_none()); - } - - fn dequeue(&mut self) -> T { - let start = self.start; - self.size -= 1; - self.start = (self.start + 1) % self.buf.len(); - let result = &mut self.buf[start]; - result.take().unwrap() - } - - fn size(&self) -> usize { - self.size - } - fn capacity(&self) -> usize { - self.buf.len() - } -} - -//////////////////////////////////////////////////////////////////////////////// -// Queue, a simple queue to enqueue threads with (stack-allocated nodes) -//////////////////////////////////////////////////////////////////////////////// - -impl Queue { - fn enqueue(&mut self, node: &mut Node) -> WaitToken { - let (wait_token, signal_token) = blocking::tokens(); - node.token = Some(signal_token); - node.next = ptr::null_mut(); - - if self.tail.is_null() { - self.head = node as *mut Node; - self.tail = node as *mut Node; - } else { - unsafe { - (*self.tail).next = node as *mut Node; - self.tail = node as *mut Node; - } - } - - wait_token - } - - fn dequeue(&mut self) -> Option<SignalToken> { - if self.head.is_null() { - return None; - } - let node = self.head; - self.head = unsafe { (*node).next }; - if self.head.is_null() { - self.tail = ptr::null_mut(); - } - unsafe { - (*node).next = ptr::null_mut(); - Some((*node).token.take().unwrap()) - } - } -} diff --git a/library/std/src/sync/mpsc/tests.rs b/library/std/src/sync/mpsc/tests.rs index f6d0796f604..82c52eb4fef 100644 --- a/library/std/src/sync/mpsc/tests.rs +++ b/library/std/src/sync/mpsc/tests.rs @@ -706,3 +706,17 @@ fn issue_32114() { let _ = tx.send(123); assert_eq!(tx.send(123), Err(SendError(123))); } + +#[test] +fn issue_39364() { + let (tx, rx) = channel::<()>(); + let t = thread::spawn(move || { + thread::sleep(Duration::from_millis(300)); + let _ = tx.clone(); + crate::mem::forget(tx); + }); + + let _ = rx.recv_timeout(Duration::from_millis(500)); + t.join().unwrap(); + let _ = rx.recv_timeout(Duration::from_millis(500)); +} |