Add futex-based RwLock on Linux.

2 files changed, 295 insertions, 2 deletions

diff --git a/library/std/src/sys/unix/locks/futex_rwlock.rs b/library/std/src/sys/unix/locks/futex_rwlock.rs
new file mode 100644
index 00000000000..0665d7b3bfd
--- /dev/null
+++ b/library/std/src/sys/unix/locks/futex_rwlock.rs
@@ -0,0 +1,293 @@
+use crate::sync::atomic::{
+    AtomicI32,
+    Ordering::{Acquire, Relaxed, Release},
+};
+use crate::sys::futex::{futex_wait, futex_wake, futex_wake_all};
+
+pub type MovableRwLock = RwLock;
+
+pub struct RwLock {
+    // The state consists of a 30-bit reader counter, a 'readers waiting' flag, and a 'writers waiting' flag.
+    // Bits 0..30:
+    //   0: Unlocked
+    //   1..=0x3FFF_FFFE: Locked by N readers
+    //   0x3FFF_FFFF: Write locked
+    // Bit 30: Readers are waiting on this futex.
+    // Bit 31: Writers are waiting on the writer_notify futex.
+    state: AtomicI32,
+    // The 'condition variable' to notify writers through.
+    // Incremented on every signal.
+    writer_notify: AtomicI32,
+}
+
+const READ_LOCKED: i32 = 1;
+const MASK: i32 = (1 << 30) - 1;
+const WRITE_LOCKED: i32 = MASK;
+const MAX_READERS: i32 = MASK - 1;
+const READERS_WAITING: i32 = 1 << 30;
+const WRITERS_WAITING: i32 = 1 << 31;
+
+fn unlocked(state: i32) -> bool {
+    state & MASK == 0
+}
+
+fn write_locked(state: i32) -> bool {
+    state & MASK == WRITE_LOCKED
+}
+
+fn readers_waiting(state: i32) -> bool {
+    state & READERS_WAITING != 0
+}
+
+fn writers_waiting(state: i32) -> bool {
+    state & WRITERS_WAITING != 0
+}
+
+fn read_lockable(state: i32) -> bool {
+    // This also returns false if the counter could overflow if we tried to read lock it.
+    state & MASK < MAX_READERS && !readers_waiting(state) && !writers_waiting(state)
+}
+
+fn reached_max_readers(state: i32) -> bool {
+    state & MASK == MAX_READERS
+}
+
+impl RwLock {
+    #[inline]
+    pub const fn new() -> Self {
+        Self { state: AtomicI32::new(0), writer_notify: AtomicI32::new(0) }
+    }
+
+    #[inline]
+    pub unsafe fn destroy(&self) {}
+
+    #[inline]
+    pub unsafe fn try_read(&self) -> bool {
+        self.state
+            .fetch_update(Acquire, Relaxed, |s| read_lockable(s).then(|| s + READ_LOCKED))
+            .is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn read(&self) {
+        if !self.try_read() {
+            self.read_contended();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn read_unlock(&self) {
+        let state = self.state.fetch_sub(READ_LOCKED, Release) - 1;
+
+        // It's impossible for a reader to be waiting on a read-locked RwLock,
+        // except if there is also a writer waiting.
+        debug_assert!(!readers_waiting(state) || writers_waiting(state));
+
+        // Wake up a writer if we were the last reader and there's a writer waiting.
+        if unlocked(state) && writers_waiting(state) {
+            self.wake_writer_or_readers(state);
+        }
+    }
+
+    #[cold]
+    fn read_contended(&self) {
+        let mut state = self.spin_read();
+
+        loop {
+            // If we can lock it, lock it.
+            if read_lockable(state) {
+                match self.state.compare_exchange(state, state + READ_LOCKED, Acquire, Relaxed) {
+                    Ok(_) => return, // Locked!
+                    Err(s) => {
+                        state = s;
+                        continue;
+                    }
+                }
+            }
+
+            // Check for overflow.
+            if reached_max_readers(state) {
+                panic!("too many active read locks on RwLock");
+            }
+
+            // Make sure the readers waiting bit is set before we go to sleep.
+            if !readers_waiting(state) {
+                if let Err(s) =
+                    self.state.compare_exchange(state, state | READERS_WAITING, Relaxed, Relaxed)
+                {
+                    state = s;
+                    continue;
+                }
+            }
+
+            // Wait for the state to change.
+            futex_wait(&self.state, state | READERS_WAITING, None);
+
+            // Spin again after waking up.
+            state = self.spin_read();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn try_write(&self) -> bool {
+        self.state.fetch_update(Acquire, Relaxed, |s| unlocked(s).then(|| s + WRITE_LOCKED)).is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn write(&self) {
+        if !self.try_write() {
+            self.write_contended();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn write_unlock(&self) {
+        let state = self.state.fetch_sub(WRITE_LOCKED, Release) - WRITE_LOCKED;
+
+        debug_assert!(unlocked(state));
+
+        if writers_waiting(state) || readers_waiting(state) {
+            self.wake_writer_or_readers(state);
+        }
+    }
+
+    #[cold]
+    fn write_contended(&self) {
+        let mut state = self.spin_write();
+
+        let mut other_writers_waiting = 0;
+
+        loop {
+            // If it's unlocked, we try to lock it.
+            if unlocked(state) {
+                match self.state.compare_exchange(
+                    state,
+                    state | WRITE_LOCKED | other_writers_waiting,
+                    Acquire,
+                    Relaxed,
+                ) {
+                    Ok(_) => return, // Locked!
+                    Err(s) => {
+                        state = s;
+                        continue;
+                    }
+                }
+            }
+
+            // Set the waiting bit indicating that we're waiting on it.
+            if !writers_waiting(state) {
+                if let Err(s) =
+                    self.state.compare_exchange(state, state | WRITERS_WAITING, Relaxed, Relaxed)
+                {
+                    state = s;
+                    continue;
+                }
+            }
+
+            // Other writers might be waiting now too, so we should make sure
+            // we keep that bit on once we manage lock it.
+            other_writers_waiting = WRITERS_WAITING;
+
+            // Examine the notification counter before we check if `state` has changed,
+            // to make sure we don't miss any notifications.
+            let seq = self.writer_notify.load(Acquire);
+
+            // Don't go to sleep if the lock has become available,
+            // or if the writers waiting bit is no longer set.
+            let s = self.state.load(Relaxed);
+            if unlocked(state) || !writers_waiting(s) {
+                state = s;
+                continue;
+            }
+
+            // Wait for the state to change.
+            futex_wait(&self.writer_notify, seq, None);
+
+            // Spin again after waking up.
+            state = self.spin_write();
+        }
+    }
+
+    /// Wake up waiting threads after unlocking.
+    ///
+    /// If both are waiting, this will wake up only one writer, but will fall
+    /// back to waking up readers if there was no writer to wake up.
+    #[cold]
+    fn wake_writer_or_readers(&self, mut state: i32) {
+        assert!(unlocked(state));
+
+        // The readers waiting bit might be turned on at any point now,
+        // since readers will block when there's anything waiting.
+        // Writers will just lock the lock though, regardless of the waiting bits,
+        // so we don't have to worry about the writer waiting bit.
+        //
+        // If the lock gets locked in the meantime, we don't have to do
+        // anything, because then the thread that locked the lock will take
+        // care of waking up waiters when it unlocks.
+
+        // If only writers are waiting, wake one of them up.
+        if state == WRITERS_WAITING {
+            match self.state.compare_exchange(state, 0, Relaxed, Relaxed) {
+                Ok(_) => {
+                    self.wake_writer();
+                    return;
+                }
+                Err(s) => {
+                    // Maybe some readers are now waiting too. So, continue to the next `if`.
+                    state = s;
+                }
+            }
+        }
+
+        // If both writers and readers are waiting, leave the readers waiting
+        // and only wake up one writer.
+        if state == READERS_WAITING + WRITERS_WAITING {
+            if self.state.compare_exchange(state, READERS_WAITING, Relaxed, Relaxed).is_err() {
+                // The lock got locked. Not our problem anymore.
+                return;
+            }
+            if self.wake_writer() {
+                return;
+            }
+            // No writers were actually waiting. Continue to wake up readers instead.
+            state = READERS_WAITING;
+        }
+
+        // If readers are waiting, wake them all up.
+        if state == READERS_WAITING {
+            if self.state.compare_exchange(state, 0, Relaxed, Relaxed).is_ok() {
+                futex_wake_all(&self.state);
+            }
+        }
+    }
+
+    fn wake_writer(&self) -> bool {
+        self.writer_notify.fetch_add(1, Release);
+        futex_wake(&self.writer_notify)
+    }
+
+    /// Spin for a while, but stop directly at the given condition.
+    fn spin_until(&self, f: impl Fn(i32) -> bool) -> i32 {
+        let mut spin = 100; // Chosen by fair dice roll.
+        loop {
+            let state = self.state.load(Relaxed);
+            if f(state) || spin == 0 {
+                return state;
+            }
+            crate::hint::spin_loop();
+            spin -= 1;
+        }
+    }
+
+    fn spin_write(&self) -> i32 {
+        // Stop spinning when it's unlocked or when there's waiting writers, to keep things somewhat fair.
+        self.spin_until(|state| unlocked(state) || writers_waiting(state))
+    }
+
+    fn spin_read(&self) -> i32 {
+        // Stop spinning when it's unlocked or read locked, or when there's waiting threads.
+        self.spin_until(|state| {
+            !write_locked(state) || readers_waiting(state) || writers_waiting(state)
+        })
+    }
+}
diff --git a/library/std/src/sys/unix/locks/mod.rs b/library/std/src/sys/unix/locks/mod.rs
index 2b8dd168068..85afc939d2e 100644
--- a/library/std/src/sys/unix/locks/mod.rs
+++ b/library/std/src/sys/unix/locks/mod.rs
@@ -4,13 +4,13 @@ cfg_if::cfg_if! {
         target_os = "android",
     ))] {
         mod futex;
+        mod futex_rwlock;
         #[allow(dead_code)]
         mod pthread_mutex; // Only used for PthreadMutexAttr, needed by pthread_remutex.
         mod pthread_remutex; // FIXME: Implement this using a futex
-        mod pthread_rwlock; // FIXME: Implement this using a futex
         pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar};
         pub use pthread_remutex::ReentrantMutex;
-        pub use pthread_rwlock::{RwLock, MovableRwLock};
+        pub use futex_rwlock::{RwLock, MovableRwLock};
     } else {
         mod pthread_mutex;
         mod pthread_remutex;