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use core::fmt; use core::cell::UnsafeCell; use core::sync::atomic::AtomicUsize; use core::sync::atomic::Ordering::{Acquire, Release, AcqRel}; use crate::task::{LocalWaker, Waker}; /// 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`. /// /// `AtomicWaker` does not provide any memory ordering guarantees, as such the /// user should use caution and use other synchronization primitives to guard /// the result of the underlying computation. 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 fn new() -> AtomicWaker { // Make sure that task is Sync trait AssertSync: Sync {} impl AssertSync for Waker {} AtomicWaker { 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. /// /// ``` /// #![feature(futures_api)] /// use futures::future::Future; /// use futures::task::{LocalWaker, Poll, AtomicWaker}; /// use std::sync::atomic::AtomicBool; /// use std::sync::atomic::Ordering::SeqCst; /// use std::pin::Pin; /// /// struct Flag { /// waker: AtomicWaker, /// set: AtomicBool, /// } /// /// impl Future for Flag { /// type Output = (); /// /// fn poll(mut self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<()> { /// // Register **before** checking `set` to avoid a race condition /// // that would result in lost notifications. /// self.waker.register(lw); /// /// if self.set.load(SeqCst) { /// Poll::Ready(()) /// } else { /// Poll::Pending /// } /// } /// } /// ``` pub fn register(&self, lw: &LocalWaker) { match self.state.compare_and_swap(WAITING, REGISTERING, Acquire) { WAITING => { unsafe { // Locked acquired, update the waker cell *self.waker.get() = Some(lw.clone().into_waker()); // Release the lock. If the state transitioned to include // the `WAKING` bit, this means that a wake has been // called concurrently, so we have to remove the waker and // wake it.` // // Start by assuming that the state is `REGISTERING` as this // is what we jut set it to. let res = self.state.compare_exchange( REGISTERING, WAITING, AcqRel, Acquire); match res { Ok(_) => {} Err(actual) => { // This branch can only be reached if a // 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(); // Just swap, because no one could change state while state == `REGISTERING` | `WAKING`. self.state.swap(WAITING, AcqRel); // The atomic swap was complete, now // wake the task and return. waker.wake(); } } } } WAKING => { // Currently in the process of waking the task, i.e., // `wake` is currently being called on the old task handle. // So, we call wake on the new waker lw.wake(); } 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. // // 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 { AtomicWaker::new() } } impl fmt::Debug for AtomicWaker { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { write!(fmt, "AtomicWaker") } } unsafe impl Send for AtomicWaker {} unsafe impl Sync for AtomicWaker {}