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use super::Task; use core::fmt; use core::cell::UnsafeCell; use core::sync::atomic::AtomicUsize; use core::sync::atomic::Ordering::{Acquire, Release, AcqRel}; /// A synchronization primitive for task notification. /// /// `AtomicTask` will coordinate concurrent notifications with the consumer /// potentially "updating" the underlying task to notify. 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 `notify` after producing the computation (this /// differs from the usual `thread::park` pattern). It is also permitted for /// `notify` to be called **before** `register`. This results in a no-op. /// /// A single `AtomicTask` may be reused for any number of calls to `register` or /// `notify`. /// /// `AtomicTask` 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 AtomicTask { state: AtomicUsize, task: UnsafeCell<Option<Task>>, } // `AtomicTask` is a multi-consumer, single-producer transfer cell. The cell // stores a `Task` value produced by calls to `register` and many threads can // race to take the task (to notify it) by calling `notify. // // If a new `Task` instance is produced by calling `register` before an existing // one is consumed, then the existing one is overwritten. // // While `AtomicTask` is single-producer, the implementation ensures memory // safety. In the event of concurrent calls to `register`, there will be a // single winner whose task will get stored in the cell. The losers will not // have their tasks notified. As such, callers should ensure to add // synchronization to calls to `register`. // // The implementation uses a single `AtomicUsize` value to coordinate access to // the `Task` cell. There are two bits that are operated on independently. These // are represented by `REGISTERING` and `NOTIFYING`. // // The `REGISTERING` bit is set when a producer enters the critical section. The // `NOTIFYING` 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 task cell by transitioning the // state from `WAITING` to `REGISTERING` or `NOTIFYING`, depending on the // operation the thread wishes to perform. When this transition is made, it is // guaranteed that no other thread will access the task 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 task cell. // // If the lock is obtained, then the thread sets the task cell to the task // 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 `notify` will observe the task. // // If the transition fails, then there was a concurrent call to `notify` that // was unable to access the task cell (due to the registering thread holding the // lock). To handle this, the registering thread removes the task it just set // from the cell and calls `notify` on it. This call to notify represents the // attempt to notify by the other thread (that set the `NOTIFYING` bit). The // state is then transitioned from `REGISTERING | NOTIFYING` back to `WAITING`. // This transition must succeed because, at this point, the state cannot be // transitioned by another thread. // // # Notifying // // On a call to `notify`, an attempt to transition the state from `WAITING` to // `NOTIFYING` is made. On success, the caller obtains a lock on the task cell. // // If the lock is obtained, then the thread takes ownership of the current value // in teh task cell, and calls `notify` 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 `NOTIFYING` 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 // `NOTIFYING` critical section. Either way, no action must be taken. // // If the current thread is the only concurrent call to `notify` and another // thread is in the `register` critical section, when the other thread **exits** // the `register` critical section, it will observe the `NOTIFYING` bit and // handle the notify itself. // // If another thread is in the `notify` critical section, then it will handle // notifying the task. // // # A potential race (is safely handled). // // Imagine the following situation: // // * Thread A obtains the `notify` lock and notifies a task. // // * Before thread A releases the `notify` lock, the notified task is scheduled. // // * Thread B attempts to notify the task. In theory this should result in the // task being notified, but it cannot because thread A still holds the notify // lock. // // This case is handled by requiring users of `AtomicTask` to call `register` // **before** attempting to observe the application state change that resulted // in the task being notified. The notifiers also change the application state // before calling notify. // // Because of this, the task will do one of two things. // // 1) Observe the application state change that Thread B is notifying on. In // this case, it is OK for Thread B's notification to be lost. // // 2) Call register before attempting to observe the application state. Since // Thread A still holds the `notify` lock, the call to `register` will result // in the task notifying itself and get scheduled again. /// Idle state const WAITING: usize = 0; /// A new task value is being registered with the `AtomicTask` cell. const REGISTERING: usize = 0b01; /// The task currently registered with the `AtomicTask` cell is being notified. const NOTIFYING: usize = 0b10; impl AtomicTask { /// Create an `AtomicTask` initialized with the given `Task` pub fn new() -> AtomicTask { // Make sure that task is Sync trait AssertSync: Sync {} impl AssertSync for Task {} AtomicTask { state: AtomicUsize::new(WAITING), task: UnsafeCell::new(None), } } /// Registers the current task to be notified on calls to `notify`. /// /// This is the same as calling `register_task` with `task::current()`. pub fn register(&self) { self.register_task(super::current()); } /// Registers the provided task to be notified on calls to `notify`. /// /// The new task will take place of any previous tasks that were registered /// by previous calls to `register`. Any calls to `notify` that happen after /// a call to `register` (as defined by the memory ordering rules), will /// notify the `register` caller's task. /// /// It is safe to call `register` with multiple other threads concurrently /// calling `notify`. 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. pub fn register_task(&self, task: Task) { match self.state.compare_and_swap(WAITING, REGISTERING, Acquire) { WAITING => { unsafe { // Locked acquired, update the waker cell *self.task.get() = Some(task.clone()); // Release the lock. If the state transitioned to include // the `NOTIFYING` bit, this means that a notify has been // called concurrently, so we have to remove the task and // notify 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 `notify`. In this // case, `actual` **must** be `REGISTERING | // `NOTIFYING`. debug_assert_eq!(actual, REGISTERING | NOTIFYING); // Take the task to notify once the atomic operation has // completed. let notify = (*self.task.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 // notify the task and return. notify.notify(); } } } } NOTIFYING => { // Currently in the process of notifying the task, i.e., // `notify` is currently being called on the old task handle. // So, we call notify on the new task handle task.notify(); } 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 | NOTIFYING); } } } /// Notifies the task that last called `register`. /// /// If `register` has not been called yet, then this does nothing. pub fn notify(&self) { // 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 `AtomicTask` is associated with. match self.state.fetch_or(NOTIFYING, AcqRel) { WAITING => { // The notifying lock has been acquired. let task = unsafe { (*self.task.get()).take() }; // Release the lock self.state.fetch_and(!NOTIFYING, Release); if let Some(task) = task { task.notify(); } } state => { // There is a concurrent thread currently updating the // associated task. // // Nothing more to do as the `NOTIFYING` bit has been set. It // doesn't matter if there are concurrent registering threads or // not. // debug_assert!( state == REGISTERING || state == REGISTERING | NOTIFYING || state == NOTIFYING); } } } } impl Default for AtomicTask { fn default() -> Self { AtomicTask::new() } } impl fmt::Debug for AtomicTask { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { write!(fmt, "AtomicTask") } } unsafe impl Send for AtomicTask {} unsafe impl Sync for AtomicTask {}