| use core::cell::UnsafeCell; |
| use core::fmt; |
| use core::task::Waker; |
| |
| use atomic::AtomicUsize; |
| use atomic::Ordering::{AcqRel, Acquire, Release}; |
| |
| #[cfg(feature = "portable-atomic")] |
| use portable_atomic as atomic; |
| |
| #[cfg(not(feature = "portable-atomic"))] |
| use core::sync::atomic; |
| |
| /// A synchronization primitive for task wakeup. |
| /// |
| /// Sometimes the task interested in a given event will change over time. |
| /// An `AtomicWaker` can coordinate concurrent notifications with the consumer |
| /// potentially "updating" the underlying task to wake up. This is useful in |
| /// scenarios where a computation completes in another thread and wants to |
| /// notify the consumer, but the consumer is in the process of being migrated to |
| /// a new logical task. |
| /// |
| /// Consumers should call `register` before checking the result of a computation |
| /// and producers should call `wake` after producing the computation (this |
| /// differs from the usual `thread::park` pattern). It is also permitted for |
| /// `wake` to be called **before** `register`. This results in a no-op. |
| /// |
| /// A single `AtomicWaker` may be reused for any number of calls to `register` or |
| /// `wake`. |
| /// |
| /// # Memory ordering |
| /// |
| /// Calling `register` "acquires" all memory "released" by calls to `wake` |
| /// before the call to `register`. Later calls to `wake` will wake the |
| /// registered waker (on contention this wake might be triggered in `register`). |
| /// |
| /// For concurrent calls to `register` (should be avoided) the ordering is only |
| /// guaranteed for the winning call. |
| /// |
| /// # Examples |
| /// |
| /// Here is a simple example providing a `Flag` that can be signalled manually |
| /// when it is ready. |
| /// |
| /// ``` |
| /// use futures::future::Future; |
| /// use futures::task::{Context, Poll, AtomicWaker}; |
| /// use std::sync::Arc; |
| /// use std::sync::atomic::AtomicBool; |
| /// use std::sync::atomic::Ordering::Relaxed; |
| /// use std::pin::Pin; |
| /// |
| /// struct Inner { |
| /// waker: AtomicWaker, |
| /// set: AtomicBool, |
| /// } |
| /// |
| /// #[derive(Clone)] |
| /// struct Flag(Arc<Inner>); |
| /// |
| /// impl Flag { |
| /// pub fn new() -> Self { |
| /// Self(Arc::new(Inner { |
| /// waker: AtomicWaker::new(), |
| /// set: AtomicBool::new(false), |
| /// })) |
| /// } |
| /// |
| /// pub fn signal(&self) { |
| /// self.0.set.store(true, Relaxed); |
| /// self.0.waker.wake(); |
| /// } |
| /// } |
| /// |
| /// impl Future for Flag { |
| /// type Output = (); |
| /// |
| /// fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> { |
| /// // quick check to avoid registration if already done. |
| /// if self.0.set.load(Relaxed) { |
| /// return Poll::Ready(()); |
| /// } |
| /// |
| /// self.0.waker.register(cx.waker()); |
| /// |
| /// // Need to check condition **after** `register` to avoid a race |
| /// // condition that would result in lost notifications. |
| /// if self.0.set.load(Relaxed) { |
| /// Poll::Ready(()) |
| /// } else { |
| /// Poll::Pending |
| /// } |
| /// } |
| /// } |
| /// ``` |
| pub struct AtomicWaker { |
| state: AtomicUsize, |
| waker: UnsafeCell<Option<Waker>>, |
| } |
| |
| // `AtomicWaker` is a multi-consumer, single-producer transfer cell. The cell |
| // stores a `Waker` value produced by calls to `register` and many threads can |
| // race to take the waker (to wake it) by calling `wake`. |
| // |
| // If a new `Waker` instance is produced by calling `register` before an |
| // existing one is consumed, then the existing one is overwritten. |
| // |
| // While `AtomicWaker` is single-producer, the implementation ensures memory |
| // safety. In the event of concurrent calls to `register`, there will be a |
| // single winner whose waker will get stored in the cell. The losers will not |
| // have their tasks woken. As such, callers should ensure to add synchronization |
| // to calls to `register`. |
| // |
| // The implementation uses a single `AtomicUsize` value to coordinate access to |
| // the `Waker` cell. There are two bits that are operated on independently. |
| // These are represented by `REGISTERING` and `WAKING`. |
| // |
| // The `REGISTERING` bit is set when a producer enters the critical section. The |
| // `WAKING` bit is set when a consumer enters the critical section. Neither bit |
| // being set is represented by `WAITING`. |
| // |
| // A thread obtains an exclusive lock on the waker cell by transitioning the |
| // state from `WAITING` to `REGISTERING` or `WAKING`, depending on the operation |
| // the thread wishes to perform. When this transition is made, it is guaranteed |
| // that no other thread will access the waker cell. |
| // |
| // # Registering |
| // |
| // On a call to `register`, an attempt to transition the state from WAITING to |
| // REGISTERING is made. On success, the caller obtains a lock on the waker cell. |
| // |
| // If the lock is obtained, then the thread sets the waker cell to the waker |
| // provided as an argument. Then it attempts to transition the state back from |
| // `REGISTERING` -> `WAITING`. |
| // |
| // If this transition is successful, then the registering process is complete |
| // and the next call to `wake` will observe the waker. |
| // |
| // If the transition fails, then there was a concurrent call to `wake` that was |
| // unable to access the waker cell (due to the registering thread holding the |
| // lock). To handle this, the registering thread removes the waker it just set |
| // from the cell and calls `wake` on it. This call to wake represents the |
| // attempt to wake by the other thread (that set the `WAKING` bit). The state is |
| // then transitioned from `REGISTERING | WAKING` back to `WAITING`. This |
| // transition must succeed because, at this point, the state cannot be |
| // transitioned by another thread. |
| // |
| // # Waking |
| // |
| // On a call to `wake`, an attempt to transition the state from `WAITING` to |
| // `WAKING` is made. On success, the caller obtains a lock on the waker cell. |
| // |
| // If the lock is obtained, then the thread takes ownership of the current value |
| // in the waker cell, and calls `wake` on it. The state is then transitioned |
| // back to `WAITING`. This transition must succeed as, at this point, the state |
| // cannot be transitioned by another thread. |
| // |
| // If the thread is unable to obtain the lock, the `WAKING` bit is still. This |
| // is because it has either been set by the current thread but the previous |
| // value included the `REGISTERING` bit **or** a concurrent thread is in the |
| // `WAKING` critical section. Either way, no action must be taken. |
| // |
| // If the current thread is the only concurrent call to `wake` and another |
| // thread is in the `register` critical section, when the other thread **exits** |
| // the `register` critical section, it will observe the `WAKING` bit and handle |
| // the wake itself. |
| // |
| // If another thread is in the `wake` critical section, then it will handle |
| // waking the task. |
| // |
| // # A potential race (is safely handled). |
| // |
| // Imagine the following situation: |
| // |
| // * Thread A obtains the `wake` lock and wakes a task. |
| // |
| // * Before thread A releases the `wake` lock, the woken task is scheduled. |
| // |
| // * Thread B attempts to wake the task. In theory this should result in the |
| // task being woken, but it cannot because thread A still holds the wake lock. |
| // |
| // This case is handled by requiring users of `AtomicWaker` to call `register` |
| // **before** attempting to observe the application state change that resulted |
| // in the task being awoken. The wakers also change the application state before |
| // calling wake. |
| // |
| // Because of this, the waker will do one of two things. |
| // |
| // 1) Observe the application state change that Thread B is woken for. In this |
| // case, it is OK for Thread B's wake to be lost. |
| // |
| // 2) Call register before attempting to observe the application state. Since |
| // Thread A still holds the `wake` lock, the call to `register` will result |
| // in the task waking itself and get scheduled again. |
| |
| /// Idle state |
| const WAITING: usize = 0; |
| |
| /// A new waker value is being registered with the `AtomicWaker` cell. |
| const REGISTERING: usize = 0b01; |
| |
| /// The waker currently registered with the `AtomicWaker` cell is being woken. |
| const WAKING: usize = 0b10; |
| |
| impl AtomicWaker { |
| /// Create an `AtomicWaker`. |
| pub const fn new() -> Self { |
| // Make sure that task is Sync |
| trait AssertSync: Sync {} |
| impl AssertSync for Waker {} |
| |
| Self { state: AtomicUsize::new(WAITING), waker: UnsafeCell::new(None) } |
| } |
| |
| /// Registers the waker to be notified on calls to `wake`. |
| /// |
| /// The new task will take place of any previous tasks that were registered |
| /// by previous calls to `register`. Any calls to `wake` that happen after |
| /// a call to `register` (as defined by the memory ordering rules), will |
| /// notify the `register` caller's task and deregister the waker from future |
| /// notifications. Because of this, callers should ensure `register` gets |
| /// invoked with a new `Waker` **each** time they require a wakeup. |
| /// |
| /// It is safe to call `register` with multiple other threads concurrently |
| /// calling `wake`. This will result in the `register` caller's current |
| /// task being notified once. |
| /// |
| /// This function is safe to call concurrently, but this is generally a bad |
| /// idea. Concurrent calls to `register` will attempt to register different |
| /// tasks to be notified. One of the callers will win and have its task set, |
| /// but there is no guarantee as to which caller will succeed. |
| /// |
| /// # Examples |
| /// |
| /// Here is how `register` is used when implementing a flag. |
| /// |
| /// ``` |
| /// use futures::future::Future; |
| /// use futures::task::{Context, Poll, AtomicWaker}; |
| /// use std::sync::atomic::AtomicBool; |
| /// use std::sync::atomic::Ordering::Relaxed; |
| /// use std::pin::Pin; |
| /// |
| /// struct Flag { |
| /// waker: AtomicWaker, |
| /// set: AtomicBool, |
| /// } |
| /// |
| /// impl Future for Flag { |
| /// type Output = (); |
| /// |
| /// fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> { |
| /// // Register **before** checking `set` to avoid a race condition |
| /// // that would result in lost notifications. |
| /// self.waker.register(cx.waker()); |
| /// |
| /// if self.set.load(Relaxed) { |
| /// Poll::Ready(()) |
| /// } else { |
| /// Poll::Pending |
| /// } |
| /// } |
| /// } |
| /// ``` |
| pub fn register(&self, waker: &Waker) { |
| match self |
| .state |
| .compare_exchange(WAITING, REGISTERING, Acquire, Acquire) |
| .unwrap_or_else(|x| x) |
| { |
| WAITING => { |
| unsafe { |
| // Locked acquired, update the waker cell |
| *self.waker.get() = Some(waker.clone()); |
| |
| // Release the lock. If the state transitioned to include |
| // the `WAKING` bit, this means that at least one wake has |
| // been called concurrently. |
| // |
| // Start by assuming that the state is `REGISTERING` as this |
| // is what we just set it to. If this holds, we know that no |
| // other writes were performed in the meantime, so there is |
| // nothing to acquire, only release. In case of concurrent |
| // wakers, we need to acquire their releases, so success needs |
| // to do both. |
| let res = self.state.compare_exchange(REGISTERING, WAITING, AcqRel, Acquire); |
| |
| match res { |
| Ok(_) => { |
| // memory ordering: acquired self.state during CAS |
| // - if previous wakes went through it syncs with |
| // their final release (`fetch_and`) |
| // - if there was no previous wake the next wake |
| // will wake us, no sync needed. |
| } |
| Err(actual) => { |
| // This branch can only be reached if at least one |
| // concurrent thread called `wake`. In this |
| // case, `actual` **must** be `REGISTERING | |
| // `WAKING`. |
| debug_assert_eq!(actual, REGISTERING | WAKING); |
| |
| // Take the waker to wake once the atomic operation has |
| // completed. |
| let waker = (*self.waker.get()).take().unwrap(); |
| |
| // We need to return to WAITING state (clear our lock and |
| // concurrent WAKING flag). This needs to acquire all |
| // WAKING fetch_or releases and it needs to release our |
| // update to self.waker, so we need a `swap` operation. |
| self.state.swap(WAITING, AcqRel); |
| |
| // memory ordering: we acquired the state for all |
| // concurrent wakes, but future wakes might still |
| // need to wake us in case we can't make progress |
| // from the pending wakes. |
| // |
| // So we simply schedule to come back later (we could |
| // also simply leave the registration in place above). |
| waker.wake(); |
| } |
| } |
| } |
| } |
| WAKING => { |
| // Currently in the process of waking the task, i.e., |
| // `wake` is currently being called on the old task handle. |
| // |
| // memory ordering: we acquired the state for all |
| // concurrent wakes, but future wakes might still |
| // need to wake us in case we can't make progress |
| // from the pending wakes. |
| // |
| // So we simply schedule to come back later (we |
| // could also spin here trying to acquire the lock |
| // to register). |
| waker.wake_by_ref(); |
| } |
| state => { |
| // In this case, a concurrent thread is holding the |
| // "registering" lock. This probably indicates a bug in the |
| // caller's code as racing to call `register` doesn't make much |
| // sense. |
| // |
| // memory ordering: don't care. a concurrent register() is going |
| // to succeed and provide proper memory ordering. |
| // |
| // We just want to maintain memory safety. It is ok to drop the |
| // call to `register`. |
| debug_assert!(state == REGISTERING || state == REGISTERING | WAKING); |
| } |
| } |
| } |
| |
| /// Calls `wake` on the last `Waker` passed to `register`. |
| /// |
| /// If `register` has not been called yet, then this does nothing. |
| pub fn wake(&self) { |
| if let Some(waker) = self.take() { |
| waker.wake(); |
| } |
| } |
| |
| /// Returns the last `Waker` passed to `register`, so that the user can wake it. |
| /// |
| /// |
| /// Sometimes, just waking the AtomicWaker is not fine grained enough. This allows the user |
| /// to take the waker and then wake it separately, rather than performing both steps in one |
| /// atomic action. |
| /// |
| /// If a waker has not been registered, this returns `None`. |
| pub fn take(&self) -> Option<Waker> { |
| // AcqRel ordering is used in order to acquire the value of the `task` |
| // cell as well as to establish a `release` ordering with whatever |
| // memory the `AtomicWaker` is associated with. |
| match self.state.fetch_or(WAKING, AcqRel) { |
| WAITING => { |
| // The waking lock has been acquired. |
| let waker = unsafe { (*self.waker.get()).take() }; |
| |
| // Release the lock |
| self.state.fetch_and(!WAKING, Release); |
| |
| waker |
| } |
| state => { |
| // There is a concurrent thread currently updating the |
| // associated task. |
| // |
| // Nothing more to do as the `WAKING` bit has been set. It |
| // doesn't matter if there are concurrent registering threads or |
| // not. |
| // |
| debug_assert!( |
| state == REGISTERING || state == REGISTERING | WAKING || state == WAKING |
| ); |
| None |
| } |
| } |
| } |
| } |
| |
| impl Default for AtomicWaker { |
| fn default() -> Self { |
| Self::new() |
| } |
| } |
| |
| impl fmt::Debug for AtomicWaker { |
| fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| write!(f, "AtomicWaker") |
| } |
| } |
| |
| unsafe impl Send for AtomicWaker {} |
| unsafe impl Sync for AtomicWaker {} |