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use crate::loom::sync::atomic::AtomicU8; use crate::loom::sync::Mutex; use crate::util::linked_list::{self, LinkedList}; use std::cell::UnsafeCell; use std::future::Future; use std::marker::PhantomPinned; use std::pin::Pin; use std::ptr::NonNull; use std::sync::atomic::Ordering::SeqCst; use std::task::{Context, Poll, Waker}; /// Notify a single task to wake up. /// /// `Notify` provides a basic mechanism to notify a single task of an event. /// `Notify` itself does not carry any data. Instead, it is to be used to signal /// another task to perform an operation. /// /// `Notify` can be thought of as a [`Semaphore`] starting with 0 permits. /// [`notified().await`] waits for a permit to become available, and [`notify()`] /// sets a permit **if there currently are no available permits**. /// /// The synchronization details of `Notify` are similar to /// [`thread::park`][park] and [`Thread::unpark`][unpark] from std. A [`Notify`] /// value contains a single permit. [`notified().await`] waits for the permit to /// be made available, consumes the permit, and resumes. [`notify()`] sets the /// permit, waking a pending task if there is one. /// /// If `notify()` is called **before** `notfied().await`, then the next call to /// `notified().await` will complete immediately, consuming the permit. Any /// subsequent calls to `notified().await` will wait for a new permit. /// /// If `notify()` is called **multiple** times before `notified().await`, only a /// **single** permit is stored. The next call to `notified().await` will /// complete immediately, but the one after will wait for a new permit. /// /// # Examples /// /// Basic usage. /// /// ``` /// use tokio::sync::Notify; /// use std::sync::Arc; /// /// #[tokio::main] /// async fn main() { /// let notify = Arc::new(Notify::new()); /// let notify2 = notify.clone(); /// /// tokio::spawn(async move { /// notify2.notified().await; /// println!("received notification"); /// }); /// /// println!("sending notification"); /// notify.notify(); /// } /// ``` /// /// Unbound mpsc channel. /// /// ``` /// use tokio::sync::Notify; /// /// use std::collections::VecDeque; /// use std::sync::Mutex; /// /// struct Channel<T> { /// values: Mutex<VecDeque<T>>, /// notify: Notify, /// } /// /// impl<T> Channel<T> { /// pub fn send(&self, value: T) { /// self.values.lock().unwrap() /// .push_back(value); /// /// // Notify the consumer a value is available /// self.notify.notify(); /// } /// /// pub async fn recv(&self) -> T { /// loop { /// // Drain values /// if let Some(value) = self.values.lock().unwrap().pop_front() { /// return value; /// } /// /// // Wait for values to be available /// self.notify.notified().await; /// } /// } /// } /// ``` /// /// [park]: std::thread::park /// [unpark]: std::thread::Thread::unpark /// [`notified().await`]: Notify::notified() /// [`notify()`]: Notify::notify() /// [`Semaphore`]: crate::sync::Semaphore #[derive(Debug)] pub struct Notify { state: AtomicU8, waiters: Mutex<LinkedList<Waiter>>, } #[derive(Debug)] struct Waiter { /// Intrusive linked-list pointers pointers: linked_list::Pointers<Waiter>, /// Waiting task's waker waker: Option<Waker>, /// `true` if the notification has been assigned to this waiter. notified: bool, /// Should not be `Unpin`. _p: PhantomPinned, } /// Future returned from `notified()` #[derive(Debug)] struct Notified<'a> { /// The `Notify` being received on. notify: &'a Notify, /// The current state of the receiving process. state: State, /// Entry in the waiter `LinkedList`. waiter: UnsafeCell<Waiter>, } unsafe impl<'a> Send for Notified<'a> {} unsafe impl<'a> Sync for Notified<'a> {} #[derive(Debug)] enum State { Init, Waiting, Done, } /// Initial "idle" state const EMPTY: u8 = 0; /// One or more threads are currently waiting to be notified. const WAITING: u8 = 1; /// Pending notification const NOTIFIED: u8 = 2; impl Notify { /// Create a new `Notify`, initialized without a permit. /// /// # Examples /// /// ``` /// use tokio::sync::Notify; /// /// let notify = Notify::new(); /// ``` pub fn new() -> Notify { Notify { state: AtomicU8::new(0), waiters: Mutex::new(LinkedList::new()), } } /// Wait for a notification. /// /// Each `Notify` value holds a single permit. If a permit is available from /// an earlier call to [`notify()`], then `notified().await` will complete /// immediately, consuming that permit. Otherwise, `notified().await` waits /// for a permit to be made available by the next call to `notify()`. /// /// [`notify()`]: Notify::notify /// /// # Examples /// /// ``` /// use tokio::sync::Notify; /// use std::sync::Arc; /// /// #[tokio::main] /// async fn main() { /// let notify = Arc::new(Notify::new()); /// let notify2 = notify.clone(); /// /// tokio::spawn(async move { /// notify2.notified().await; /// println!("received notification"); /// }); /// /// println!("sending notification"); /// notify.notify(); /// } /// ``` pub async fn notified(&self) { Notified { notify: self, state: State::Init, waiter: UnsafeCell::new(Waiter { pointers: linked_list::Pointers::new(), waker: None, notified: false, _p: PhantomPinned, }), } .await } /// Notifies a waiting task /// /// If a task is currently waiting, that task is notified. Otherwise, a /// permit is stored in this `Notify` value and the **next** call to /// [`notified().await`] will complete immediately consuming the permit made /// available by this call to `notify()`. /// /// At most one permit may be stored by `Notify`. Many sequential calls to /// `notify` will result in a single permit being stored. The next call to /// `notified().await` will complete immediately, but the one after that /// will wait. /// /// [`notified().await`]: Notify::notified() /// /// # Examples /// /// ``` /// use tokio::sync::Notify; /// use std::sync::Arc; /// /// #[tokio::main] /// async fn main() { /// let notify = Arc::new(Notify::new()); /// let notify2 = notify.clone(); /// /// tokio::spawn(async move { /// notify2.notified().await; /// println!("received notification"); /// }); /// /// println!("sending notification"); /// notify.notify(); /// } /// ``` pub fn notify(&self) { // Load the current state let mut curr = self.state.load(SeqCst); // If the state is `EMPTY`, transition to `NOTIFIED` and return. while let EMPTY | NOTIFIED = curr { // The compare-exchange from `NOTIFIED` -> `NOTIFIED` is intended. A // happens-before synchronization must happen between this atomic // operation and a task calling `notified().await`. let res = self.state.compare_exchange(curr, NOTIFIED, SeqCst, SeqCst); match res { // No waiters, no further work to do Ok(_) => return, Err(actual) => { curr = actual; } } } // There are waiters, the lock must be acquired to notify. let mut waiters = self.waiters.lock().unwrap(); // The state must be reloaded while the lock is held. The state may only // transition out of WAITING while the lock is held. curr = self.state.load(SeqCst); if let Some(waker) = notify_locked(&mut waiters, &self.state, curr) { drop(waiters); waker.wake(); } } } impl Default for Notify { fn default() -> Notify { Notify::new() } } fn notify_locked(waiters: &mut LinkedList<Waiter>, state: &AtomicU8, curr: u8) -> Option<Waker> { loop { match curr { EMPTY | NOTIFIED => { let res = state.compare_exchange(curr, NOTIFIED, SeqCst, SeqCst); match res { Ok(_) => return None, Err(actual) => { assert!(actual == EMPTY || actual == NOTIFIED); state.store(NOTIFIED, SeqCst); return None; } } } WAITING => { // At this point, it is guaranteed that the state will not // concurrently change as holding the lock is required to // transition **out** of `WAITING`. // // Get a pending waiter let mut waiter = waiters.pop_back().unwrap(); // Safety: `waiters` lock is still held. let waiter = unsafe { waiter.as_mut() }; assert!(!waiter.notified); waiter.notified = true; let waker = waiter.waker.take(); if waiters.is_empty() { // As this the **final** waiter in the list, the state // must be transitioned to `EMPTY`. As transitioning // **from** `WAITING` requires the lock to be held, a // `store` is sufficient. state.store(EMPTY, SeqCst); } return waker; } _ => unreachable!(), } } } // ===== impl Notified ===== impl Notified<'_> { /// A custom `project` implementation is used in place of `pin-project-lite` /// as a custom drop implementation is needed. fn project(self: Pin<&mut Self>) -> (&Notify, &mut State, &UnsafeCell<Waiter>) { unsafe { // Safety: both `notify` and `state` are `Unpin`. is_unpin::<&Notify>(); is_unpin::<AtomicU8>(); let me = self.get_unchecked_mut(); (&me.notify, &mut me.state, &me.waiter) } } } impl Future for Notified<'_> { type Output = (); fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> { use State::*; let (notify, state, waiter) = self.project(); loop { match *state { Init => { // Optimistically try acquiring a pending notification let res = notify .state .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst); if res.is_ok() { // Acquired the notification *state = Done; return Poll::Ready(()); } // Acquire the lock and attempt to transition to the waiting // state. let mut waiters = notify.waiters.lock().unwrap(); // Reload the state with the lock held let mut curr = notify.state.load(SeqCst); // Transition the state to WAITING. loop { match curr { EMPTY => { // Transition to WAITING let res = notify .state .compare_exchange(EMPTY, WAITING, SeqCst, SeqCst); if let Err(actual) = res { assert_eq!(actual, NOTIFIED); curr = actual; } else { break; } } WAITING => break, NOTIFIED => { // Try consuming the notification let res = notify .state .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst); match res { Ok(_) => { // Acquired the notification *state = Done; return Poll::Ready(()); } Err(actual) => { assert_eq!(actual, EMPTY); curr = actual; } } } _ => unreachable!(), } } // Safety: called while locked. unsafe { (*waiter.get()).waker = Some(cx.waker().clone()); } // Insert the waiter into the linked list // // safety: pointers from `UnsafeCell` are never null. waiters.push_front(unsafe { NonNull::new_unchecked(waiter.get()) }); *state = Waiting; } Waiting => { // Currently in the "Waiting" state, implying the caller has // a waiter stored in the waiter list (guarded by // `notify.waiters`). In order to access the waker fields, // we must hold the lock. let waiters = notify.waiters.lock().unwrap(); // Safety: called while locked let w = unsafe { &mut *waiter.get() }; if w.notified { // Our waker has been notified. Reset the fields and // remove it from the list. w.waker = None; w.notified = false; *state = Done; } else { // Update the waker, if necessary. if !w.waker.as_ref().unwrap().will_wake(cx.waker()) { w.waker = Some(cx.waker().clone()); } return Poll::Pending; } // Explicit drop of the lock to indicate the scope that the // lock is held. Because holding the lock is required to // ensure safe access to fields not held within the lock, it // is helpful to visualize the scope of the critical // section. drop(waiters); } Done => { return Poll::Ready(()); } } } } } impl Drop for Notified<'_> { fn drop(&mut self) { use State::*; // Safety: The type only transitions to a "Waiting" state when pinned. let (notify, state, waiter) = unsafe { Pin::new_unchecked(self).project() }; // This is where we ensure safety. The `Notified` value is being // dropped, which means we must ensure that the waiter entry is no // longer stored in the linked list. if let Waiting = *state { let mut notify_state = WAITING; let mut waiters = notify.waiters.lock().unwrap(); // `Notify.state` may be in any of the three states (Empty, Waiting, // Notified). It doesn't actually matter what the atomic is set to // at this point. We hold the lock and will ensure the atomic is in // the correct state once th elock is dropped. // // Because the atomic state is not checked, at first glance, it may // seem like this routine does not handle the case where the // receiver is notified but has not yet observed the notification. // If this happens, no matter how many notifications happen between // this receiver being notified and the receive future dropping, all // we need to do is ensure that one notification is returned back to // the `Notify`. This is done by calling `notify_locked` if `self` // has the `notified` flag set. // remove the entry from the list // // safety: the waiter is only added to `waiters` by virtue of it // being the only `LinkedList` available to the type. unsafe { waiters.remove(NonNull::new_unchecked(waiter.get())) }; if waiters.is_empty() { notify_state = EMPTY; // If the state *should* be `NOTIFIED`, the call to // `notify_locked` below will end up doing the // `store(NOTIFIED)`. If a concurrent receiver races and // observes the incorrect `EMPTY` state, it will then obtain the // lock and block until `notify.state` is in the correct final // state. notify.state.store(EMPTY, SeqCst); } // See if the node was notified but not received. In this case, the // notification must be sent to another waiter. // // Safety: with the entry removed from the linked list, there can be // no concurrent access to the entry let notified = unsafe { (*waiter.get()).notified }; if notified { if let Some(waker) = notify_locked(&mut waiters, ¬ify.state, notify_state) { drop(waiters); waker.wake(); } } } } } /// # Safety /// /// `Waiter` is forced to be !Unpin. unsafe impl linked_list::Link for Waiter { type Handle = NonNull<Waiter>; type Target = Waiter; fn as_raw(handle: &NonNull<Waiter>) -> NonNull<Waiter> { *handle } unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> { ptr } unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> { NonNull::from(&mut target.as_mut().pointers) } } fn is_unpin<T: Unpin>() {}