| #![cfg_attr(any(loom, not(feature = "sync")), allow(dead_code, unreachable_pub))] |
| |
| use crate::loom::cell::UnsafeCell; |
| use crate::loom::sync::atomic::{self, AtomicUsize}; |
| |
| use std::fmt; |
| use std::sync::atomic::Ordering::{AcqRel, Acquire, Release}; |
| use std::task::Waker; |
| |
| /// A synchronization primitive for task waking. |
| /// |
| /// `AtomicWaker` will coordinate concurrent wakes with the consumer |
| /// potentially "waking" the underlying task. This is useful in scenarios |
| /// where a computation completes in another thread and wants to wake 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`. |
| pub(crate) 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 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 waker itself. |
| // |
| // If another thread is in the `waker` critical section, then it will handle |
| // waking the caller 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 woken. The wakers also change the application state |
| // before calling wake. |
| // |
| // Because of this, the task will do one of two things. |
| // |
| // 1) Observe the application state change that Thread B is waking on. 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 task currently registered with the `AtomicWaker` cell is being woken. |
| const WAKING: usize = 0b10; |
| |
| impl AtomicWaker { |
| /// Create an `AtomicWaker` |
| pub(crate) fn new() -> AtomicWaker { |
| AtomicWaker { |
| state: AtomicUsize::new(WAITING), |
| waker: UnsafeCell::new(None), |
| } |
| } |
| |
| /* |
| /// Registers the current waker to be notified on calls to `wake`. |
| pub(crate) fn register(&self, waker: Waker) { |
| self.do_register(waker); |
| } |
| */ |
| |
| /// Registers the provided waker to be notified on calls to `wake`. |
| /// |
| /// The new waker will take place of any previous wakers 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 |
| /// wake the `register` caller's task. |
| /// |
| /// It is safe to call `register` with multiple other threads concurrently |
| /// calling `wake`. This will result in the `register` caller's current |
| /// task being woken 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 woken. One of the callers will win and have its task set, |
| /// but there is no guarantee as to which caller will succeed. |
| pub(crate) fn register_by_ref(&self, waker: &Waker) { |
| self.do_register(waker); |
| } |
| |
| fn do_register<W>(&self, waker: W) |
| where |
| W: WakerRef, |
| { |
| match self |
| .state |
| .compare_exchange(WAITING, REGISTERING, Acquire, Acquire) |
| .unwrap_or_else(|x| x) |
| { |
| WAITING => { |
| unsafe { |
| // Locked acquired, update the waker cell |
| self.waker.with_mut(|t| *t = Some(waker.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.with_mut(|t| (*t).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 waker and return. |
| waker.wake(); |
| } |
| } |
| } |
| } |
| WAKING => { |
| // Currently in the process of waking the task, i.e., |
| // `wake` is currently being called on the old waker. |
| // So, we call wake on the new waker. |
| waker.wake(); |
| |
| // This is equivalent to a spin lock, so use a spin hint. |
| // TODO: once we bump MSRV to 1.49+, use `hint::spin_loop` instead. |
| #[allow(deprecated)] |
| atomic::spin_loop_hint(); |
| } |
| 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); |
| } |
| } |
| } |
| |
| /// Wakes the task that last called `register`. |
| /// |
| /// If `register` has not been called yet, then this does nothing. |
| pub(crate) fn wake(&self) { |
| if let Some(waker) = self.take_waker() { |
| waker.wake(); |
| } |
| } |
| |
| /// Attempts to take the `Waker` value out of the `AtomicWaker` with the |
| /// intention that the caller will wake the task later. |
| pub(crate) fn take_waker(&self) -> Option<Waker> { |
| // AcqRel ordering is used in order to acquire the value of the `waker` |
| // 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.with_mut(|t| (*t).take()) }; |
| |
| // Release the lock |
| self.state.fetch_and(!WAKING, Release); |
| |
| waker |
| } |
| state => { |
| // There is a concurrent thread currently updating the |
| // associated waker. |
| // |
| // 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 {} |
| |
| trait WakerRef { |
| fn wake(self); |
| fn into_waker(self) -> Waker; |
| } |
| |
| impl WakerRef for Waker { |
| fn wake(self) { |
| self.wake() |
| } |
| |
| fn into_waker(self) -> Waker { |
| self |
| } |
| } |
| |
| impl WakerRef for &Waker { |
| fn wake(self) { |
| self.wake_by_ref() |
| } |
| |
| fn into_waker(self) -> Waker { |
| self.clone() |
| } |
| } |