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#![cfg_attr(any(loom, not(feature = "sync")), allow(dead_code, unreachable_pub))]
use crate::loom::cell::UnsafeCell;
use crate::loom::hint;
use crate::loom::sync::atomic::AtomicUsize;
use std::fmt;
use std::panic::{resume_unwind, AssertUnwindSafe, RefUnwindSafe, UnwindSafe};
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>>,
}
impl RefUnwindSafe for AtomicWaker {}
impl UnwindSafe for AtomicWaker {}
// `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 set.
// 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,
{
fn catch_unwind<F: FnOnce() -> R, R>(f: F) -> std::thread::Result<R> {
std::panic::catch_unwind(AssertUnwindSafe(f))
}
match self
.state
.compare_exchange(WAITING, REGISTERING, Acquire, Acquire)
.unwrap_or_else(|x| x)
{
WAITING => {
unsafe {
// If `into_waker` panics (because it's code outside of
// AtomicWaker) we need to prime a guard that is called on
// unwind to restore the waker to a WAITING state. Otherwise
// any future calls to register will incorrectly be stuck
// believing it's being updated by someone else.
let new_waker_or_panic = catch_unwind(move || waker.into_waker());
// Set the field to contain the new waker, or if
// `into_waker` panicked, leave the old value.
let mut maybe_panic = None;
let mut old_waker = None;
match new_waker_or_panic {
Ok(new_waker) => {
old_waker = self.waker.with_mut(|t| (*t).take());
self.waker.with_mut(|t| *t = Some(new_waker));
}
Err(panic) => maybe_panic = Some(panic),
}
// 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(_) => {
// We don't want to give the caller the panic if it
// was someone else who put in that waker.
let _ = catch_unwind(move || {
drop(old_waker);
});
}
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 mut waker = self.waker.with_mut(|t| (*t).take());
// Just swap, because no one could change state
// while state == `Registering | `Waking`
self.state.swap(WAITING, AcqRel);
// If `into_waker` panicked, then the waker in the
// waker slot is actually the old waker.
if maybe_panic.is_some() {
old_waker = waker.take();
}
// We don't want to give the caller the panic if it
// was someone else who put in that waker.
if let Some(old_waker) = old_waker {
let _ = catch_unwind(move || {
old_waker.wake();
});
}
// The atomic swap was complete, now wake the waker
// and return.
//
// If this panics, we end up in a consumed state and
// return the panic to the caller.
if let Some(waker) = waker {
debug_assert!(maybe_panic.is_none());
waker.wake();
}
}
}
if let Some(panic) = maybe_panic {
// If `into_waker` panicked, return the panic to the caller.
resume_unwind(panic);
}
}
}
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.
//
// If this panics, someone else is responsible for restoring the
// state of the waker.
waker.wake();
// This is equivalent to a spin lock, so use a spin hint.
hint::spin_loop();
}
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() {
// If wake panics, we've consumed the waker which is a legitimate
// outcome.
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()
}
}