diff options
| -rw-r--r-- | library/std/src/sys/windows/mod.rs | 1 | ||||
| -rw-r--r-- | library/std/src/sys/windows/thread_parker.rs | 178 | ||||
| -rw-r--r-- | library/std/src/sys_common/thread_parker/mod.rs | 2 |
3 files changed, 181 insertions, 0 deletions
diff --git a/library/std/src/sys/windows/mod.rs b/library/std/src/sys/windows/mod.rs index 8178e6806b9..1aece863286 100644 --- a/library/std/src/sys/windows/mod.rs +++ b/library/std/src/sys/windows/mod.rs @@ -36,6 +36,7 @@ pub mod rwlock; pub mod thread; pub mod thread_local_dtor; pub mod thread_local_key; +pub mod thread_parker; pub mod time; cfg_if::cfg_if! { if #[cfg(not(target_vendor = "uwp"))] { diff --git a/library/std/src/sys/windows/thread_parker.rs b/library/std/src/sys/windows/thread_parker.rs new file mode 100644 index 00000000000..68a060ba32d --- /dev/null +++ b/library/std/src/sys/windows/thread_parker.rs @@ -0,0 +1,178 @@ +use crate::convert::TryFrom; +use crate::ptr; +use crate::sync::atomic::{ + AtomicI8, AtomicUsize, + Ordering::{Acquire, Relaxed, Release}, +}; +use crate::sys::{c, dur2timeout}; +use crate::time::Duration; + +pub struct Parker { + state: AtomicI8, +} + +const PARKED: i8 = -1; +const EMPTY: i8 = 0; +const NOTIFIED: i8 = 1; + +// Notes about memory ordering: +// +// Memory ordering is only relevant for the relative ordering of operations +// between different variables. Even Ordering::Relaxed guarantees a +// monotonic/consistent order when looking at just a single atomic variable. +// +// So, since this parker is just a single atomic variable, we only need to look +// at the ordering guarantees we need to provide to the 'outside world'. +// +// The only memory ordering guarantee that parking and unparking provide, is +// that things which happened before unpark() are visible on the thread +// returning from park() afterwards. Otherwise, it was effectively unparked +// before unpark() was called while still consuming the 'token'. +// +// In other words, unpark() needs to synchronize with the part of park() that +// consumes the token and returns. +// +// This is done with a release-acquire synchronization, by using +// Ordering::Release when writing NOTIFIED (the 'token') in unpark(), and using +// Ordering::Acquire when checking for this state in park(). +impl Parker { + pub fn new() -> Self { + Self { state: AtomicI8::new(EMPTY) } + } + + // Assumes this is only called by the thread that owns the Parker, + // which means that `self.state != PARKED`. + pub unsafe fn park(&self) { + // Change NOTIFIED=>EMPTY or EMPTY=>PARKED, and directly return in the + // first case. + if self.state.fetch_sub(1, Acquire) == NOTIFIED { + return; + } + + loop { + // Wait for something to happen. + if c::WaitOnAddress::is_available() { + c::WaitOnAddress(self.ptr(), &PARKED as *const _ as c::LPVOID, 1, c::INFINITE); + } else { + c::NtWaitForKeyedEvent(keyed_event_handle(), self.ptr(), 0, ptr::null_mut()); + } + // Change NOTIFIED=>EMPTY and return in that case. + if self.state.compare_and_swap(NOTIFIED, EMPTY, Acquire) == NOTIFIED { + return; + } else { + // Spurious wake up. We loop to try again. + } + } + } + + // Assumes this is only called by the thread that owns the Parker, + // which means that `self.state != PARKED`. + pub unsafe fn park_timeout(&self, timeout: Duration) { + // Change NOTIFIED=>EMPTY or EMPTY=>PARKED, and directly return in the + // first case. + if self.state.fetch_sub(1, Acquire) == NOTIFIED { + return; + } + + if c::WaitOnAddress::is_available() { + // Wait for something to happen, assuming it's still set to PARKED. + c::WaitOnAddress(self.ptr(), &PARKED as *const _ as c::LPVOID, 1, dur2timeout(timeout)); + // Change NOTIFIED=>EMPTY and return in that case. + if self.state.swap(EMPTY, Acquire) == NOTIFIED { + return; + } else { + // Timeout or spurious wake up. + // We return either way, because we can't easily tell if it was the + // timeout or not. + } + } else { + // Need to wait for unpark() using NtWaitForKeyedEvent. + let handle = keyed_event_handle(); + + // NtWaitForKeyedEvent uses a unit of 100ns, and uses negative values + // to indicate the monotonic clock. + let mut timeout = match i64::try_from((timeout.as_nanos() + 99) / 100) { + Ok(t) => -t, + Err(_) => i64::MIN, + }; + + // Wait for unpark() to produce this event. + if c::NtWaitForKeyedEvent(handle, self.ptr(), 0, &mut timeout) == c::STATUS_SUCCESS { + // Awoken by another thread. + self.state.swap(EMPTY, Acquire); + } else { + // Not awoken by another thread (spurious or timeout). + if self.state.swap(EMPTY, Acquire) == NOTIFIED { + // If the state is NOTIFIED, we *just* missed an unpark(), + // which is now waiting for us to wait for it. + // Wait for it to consume the event and unblock it. + c::NtWaitForKeyedEvent(handle, self.ptr(), 0, ptr::null_mut()); + } + } + } + } + + pub fn unpark(&self) { + // Change PARKED=>NOTIFIED, EMPTY=>NOTIFIED, or NOTIFIED=>NOTIFIED, and + // wake the thread in the first case. + // + // Note that even NOTIFIED=>NOTIFIED results in a write. This is on + // purpose, to make sure every unpark() has a release-acquire ordering + // with park(). + if self.state.swap(NOTIFIED, Release) == PARKED { + if c::WakeByAddressSingle::is_available() { + unsafe { + c::WakeByAddressSingle(self.ptr()); + } + } else { + // If we run NtReleaseKeyedEvent before the waiting thread runs + // NtWaitForKeyedEvent, this (shortly) blocks until we can wake it up. + // If the waiting thread wakes up before we run NtReleaseKeyedEvent + // (e.g. due to a timeout), this blocks until we do wake up a thread. + // To prevent this thread from blocking indefinitely in that case, + // park_impl() will, after seeing the state set to NOTIFIED after + // waking up, call NtWaitForKeyedEvent again to unblock us. + unsafe { + c::NtReleaseKeyedEvent(keyed_event_handle(), self.ptr(), 0, ptr::null_mut()); + } + } + } + } + + fn ptr(&self) -> c::LPVOID { + &self.state as *const _ as c::LPVOID + } +} + +fn keyed_event_handle() -> c::HANDLE { + const INVALID: usize = !0; + static HANDLE: AtomicUsize = AtomicUsize::new(INVALID); + match HANDLE.load(Relaxed) { + INVALID => { + let mut handle = c::INVALID_HANDLE_VALUE; + unsafe { + match c::NtCreateKeyedEvent( + &mut handle, + c::GENERIC_READ | c::GENERIC_WRITE, + ptr::null_mut(), + 0, + ) { + c::STATUS_SUCCESS => {} + r => panic!("Unable to create keyed event handle: error {}", r), + } + } + match HANDLE.compare_exchange(INVALID, handle as usize, Relaxed, Relaxed) { + Ok(_) => handle, + Err(h) => { + // Lost the race to another thread initializing HANDLE before we did. + // Closing our handle and using theirs instead. + unsafe { + c::CloseHandle(handle); + } + h as c::HANDLE + } + } + } + handle => handle as c::HANDLE, + } +} diff --git a/library/std/src/sys_common/thread_parker/mod.rs b/library/std/src/sys_common/thread_parker/mod.rs index 23c17c8e2cf..92b17ef4430 100644 --- a/library/std/src/sys_common/thread_parker/mod.rs +++ b/library/std/src/sys_common/thread_parker/mod.rs @@ -2,6 +2,8 @@ cfg_if::cfg_if! { if #[cfg(any(target_os = "linux", target_os = "android"))] { mod futex; pub use futex::Parker; + } else if #[cfg(windows)] { + pub use crate::sys::thread_parker::Parker; } else { mod generic; pub use generic::Parker; |