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//! Notify async tasks or threads.
//!
//! This is a synchronization primitive similar to [eventcounts] invented by Dmitry Vyukov.
//!
//! You can use this crate to turn non-blocking data structures into async or blocking data
//! structures. See a [simple mutex] implementation that exposes an async and a blocking interface
//! for acquiring locks.
//!
//! [eventcounts]: http://www.1024cores.net/home/lock-free-algorithms/eventcounts
//! [simple mutex]: https://github.com/smol-rs/event-listener/blob/master/examples/mutex.rs
//!
//! # Examples
//!
//! Wait until another thread sets a boolean flag:
//!
//! ```
//! use std::sync::atomic::{AtomicBool, Ordering};
//! use std::sync::Arc;
//! use std::thread;
//! use std::time::Duration;
//! use std::usize;
//! use event_listener::Event;
//!
//! let flag = Arc::new(AtomicBool::new(false));
//! let event = Arc::new(Event::new());
//!
//! // Spawn a thread that will set the flag after 1 second.
//! thread::spawn({
//! let flag = flag.clone();
//! let event = event.clone();
//! move || {
//! // Wait for a second.
//! thread::sleep(Duration::from_secs(1));
//!
//! // Set the flag.
//! flag.store(true, Ordering::SeqCst);
//!
//! // Notify all listeners that the flag has been set.
//! event.notify(usize::MAX);
//! }
//! });
//!
//! // Wait until the flag is set.
//! loop {
//! // Check the flag.
//! if flag.load(Ordering::SeqCst) {
//! break;
//! }
//!
//! // Start listening for events.
//! let listener = event.listen();
//!
//! // Check the flag again after creating the listener.
//! if flag.load(Ordering::SeqCst) {
//! break;
//! }
//!
//! // Wait for a notification and continue the loop.
//! listener.wait();
//! }
//! ```
#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
use std::cell::{Cell, UnsafeCell};
use std::fmt;
use std::future::Future;
use std::mem::{self, ManuallyDrop};
use std::ops::{Deref, DerefMut};
use std::panic::{RefUnwindSafe, UnwindSafe};
use std::pin::Pin;
use std::ptr::{self, NonNull};
use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
use std::sync::{Arc, Mutex, MutexGuard};
use std::task::{Context, Poll, Waker};
use std::thread::{self, Thread};
use std::time::{Duration, Instant};
use std::usize;
/// Inner state of [`Event`].
struct Inner {
/// The number of notified entries, or `usize::MAX` if all of them have been notified.
///
/// If there are no entries, this value is set to `usize::MAX`.
notified: AtomicUsize,
/// A linked list holding registered listeners.
list: Mutex<List>,
/// A single cached list entry to avoid allocations on the fast path of the insertion.
cache: UnsafeCell<Entry>,
}
impl Inner {
/// Locks the list.
fn lock(&self) -> ListGuard<'_> {
ListGuard {
inner: self,
guard: self.list.lock().unwrap(),
}
}
/// Returns the pointer to the single cached list entry.
#[inline(always)]
fn cache_ptr(&self) -> NonNull<Entry> {
unsafe { NonNull::new_unchecked(self.cache.get()) }
}
}
/// A synchronization primitive for notifying async tasks and threads.
///
/// Listeners can be registered using [`Event::listen()`]. There are two ways to notify listeners:
///
/// 1. [`Event::notify()`] notifies a number of listeners.
/// 2. [`Event::notify_additional()`] notifies a number of previously unnotified listeners.
///
/// If there are no active listeners at the time a notification is sent, it simply gets lost.
///
/// There are two ways for a listener to wait for a notification:
///
/// 1. In an asynchronous manner using `.await`.
/// 2. In a blocking manner by calling [`EventListener::wait()`] on it.
///
/// If a notified listener is dropped without receiving a notification, dropping will notify
/// another active listener. Whether one *additional* listener will be notified depends on what
/// kind of notification was delivered.
///
/// Listeners are registered and notified in the first-in first-out fashion, ensuring fairness.
pub struct Event {
/// A pointer to heap-allocated inner state.
///
/// This pointer is initially null and gets lazily initialized on first use. Semantically, it
/// is an `Arc<Inner>` so it's important to keep in mind that it contributes to the [`Arc`]'s
/// reference count.
inner: AtomicPtr<Inner>,
}
unsafe impl Send for Event {}
unsafe impl Sync for Event {}
impl UnwindSafe for Event {}
impl RefUnwindSafe for Event {}
impl Event {
/// Creates a new [`Event`].
///
/// # Examples
///
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
/// ```
#[inline]
pub const fn new() -> Event {
Event {
inner: AtomicPtr::new(ptr::null_mut()),
}
}
/// Returns a guard listening for a notification.
///
/// This method emits a `SeqCst` fence after registering a listener.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
/// let listener = event.listen();
/// ```
#[cold]
pub fn listen(&self) -> EventListener {
let inner = self.inner();
let listener = EventListener {
inner: unsafe { Arc::clone(&ManuallyDrop::new(Arc::from_raw(inner))) },
entry: unsafe { Some((*inner).lock().insert((*inner).cache_ptr())) },
};
// Make sure the listener is registered before whatever happens next.
full_fence();
listener
}
/// Notifies a number of active listeners.
///
/// The number is allowed to be zero or exceed the current number of listeners.
///
/// In contrast to [`Event::notify_additional()`], this method only makes sure *at least* `n`
/// listeners among the active ones are notified.
///
/// This method emits a `SeqCst` fence before notifying listeners.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
///
/// // This notification gets lost because there are no listeners.
/// event.notify(1);
///
/// let listener1 = event.listen();
/// let listener2 = event.listen();
/// let listener3 = event.listen();
///
/// // Notifies two listeners.
/// //
/// // Listener queueing is fair, which means `listener1` and `listener2`
/// // get notified here since they start listening before `listener3`.
/// event.notify(2);
/// ```
#[inline]
pub fn notify(&self, n: usize) {
// Make sure the notification comes after whatever triggered it.
full_fence();
if let Some(inner) = self.try_inner() {
// Notify if there is at least one unnotified listener and the number of notified
// listeners is less than `n`.
if inner.notified.load(Ordering::Acquire) < n {
inner.lock().notify(n);
}
}
}
/// Notifies a number of active listeners without emitting a `SeqCst` fence.
///
/// The number is allowed to be zero or exceed the current number of listeners.
///
/// In contrast to [`Event::notify_additional()`], this method only makes sure *at least* `n`
/// listeners among the active ones are notified.
///
/// Unlike [`Event::notify()`], this method does not emit a `SeqCst` fence.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
/// use std::sync::atomic::{self, Ordering};
///
/// let event = Event::new();
///
/// // This notification gets lost because there are no listeners.
/// event.notify(1);
///
/// let listener1 = event.listen();
/// let listener2 = event.listen();
/// let listener3 = event.listen();
///
/// // We should emit a fence manually when using relaxed notifications.
/// atomic::fence(Ordering::SeqCst);
///
/// // Notifies two listeners.
/// //
/// // Listener queueing is fair, which means `listener1` and `listener2`
/// // get notified here since they start listening before `listener3`.
/// event.notify(2);
/// ```
#[inline]
pub fn notify_relaxed(&self, n: usize) {
if let Some(inner) = self.try_inner() {
// Notify if there is at least one unnotified listener and the number of notified
// listeners is less than `n`.
if inner.notified.load(Ordering::Acquire) < n {
inner.lock().notify(n);
}
}
}
/// Notifies a number of active and still unnotified listeners.
///
/// The number is allowed to be zero or exceed the current number of listeners.
///
/// In contrast to [`Event::notify()`], this method will notify `n` *additional* listeners that
/// were previously unnotified.
///
/// This method emits a `SeqCst` fence before notifying listeners.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
///
/// // This notification gets lost because there are no listeners.
/// event.notify(1);
///
/// let listener1 = event.listen();
/// let listener2 = event.listen();
/// let listener3 = event.listen();
///
/// // Notifies two listeners.
/// //
/// // Listener queueing is fair, which means `listener1` and `listener2`
/// // get notified here since they start listening before `listener3`.
/// event.notify_additional(1);
/// event.notify_additional(1);
/// ```
#[inline]
pub fn notify_additional(&self, n: usize) {
// Make sure the notification comes after whatever triggered it.
full_fence();
if let Some(inner) = self.try_inner() {
// Notify if there is at least one unnotified listener.
if inner.notified.load(Ordering::Acquire) < usize::MAX {
inner.lock().notify_additional(n);
}
}
}
/// Notifies a number of active and still unnotified listeners without emitting a `SeqCst`
/// fence.
///
/// The number is allowed to be zero or exceed the current number of listeners.
///
/// In contrast to [`Event::notify()`], this method will notify `n` *additional* listeners that
/// were previously unnotified.
///
/// Unlike [`Event::notify_additional()`], this method does not emit a `SeqCst` fence.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
/// use std::sync::atomic::{self, Ordering};
///
/// let event = Event::new();
///
/// // This notification gets lost because there are no listeners.
/// event.notify(1);
///
/// let listener1 = event.listen();
/// let listener2 = event.listen();
/// let listener3 = event.listen();
///
/// // We should emit a fence manually when using relaxed notifications.
/// atomic::fence(Ordering::SeqCst);
///
/// // Notifies two listeners.
/// //
/// // Listener queueing is fair, which means `listener1` and `listener2`
/// // get notified here since they start listening before `listener3`.
/// event.notify_additional_relaxed(1);
/// event.notify_additional_relaxed(1);
/// ```
#[inline]
pub fn notify_additional_relaxed(&self, n: usize) {
if let Some(inner) = self.try_inner() {
// Notify if there is at least one unnotified listener.
if inner.notified.load(Ordering::Acquire) < usize::MAX {
inner.lock().notify_additional(n);
}
}
}
/// Returns a reference to the inner state if it was initialized.
#[inline]
fn try_inner(&self) -> Option<&Inner> {
let inner = self.inner.load(Ordering::Acquire);
unsafe { inner.as_ref() }
}
/// Returns a raw pointer to the inner state, initializing it if necessary.
///
/// This returns a raw pointer instead of reference because `from_raw`
/// requires raw/mut provenance: <https://github.com/rust-lang/rust/pull/67339>
fn inner(&self) -> *const Inner {
let mut inner = self.inner.load(Ordering::Acquire);
// Initialize the state if this is its first use.
if inner.is_null() {
// Allocate on the heap.
let new = Arc::new(Inner {
notified: AtomicUsize::new(usize::MAX),
list: std::sync::Mutex::new(List {
head: None,
tail: None,
start: None,
len: 0,
notified: 0,
cache_used: false,
}),
cache: UnsafeCell::new(Entry {
state: Cell::new(State::Created),
prev: Cell::new(None),
next: Cell::new(None),
}),
});
// Convert the heap-allocated state into a raw pointer.
let new = Arc::into_raw(new) as *mut Inner;
// Attempt to replace the null-pointer with the new state pointer.
inner = self
.inner
.compare_exchange(inner, new, Ordering::AcqRel, Ordering::Acquire)
.unwrap_or_else(|x| x);
// Check if the old pointer value was indeed null.
if inner.is_null() {
// If yes, then use the new state pointer.
inner = new;
} else {
// If not, that means a concurrent operation has initialized the state.
// In that case, use the old pointer and deallocate the new one.
unsafe {
drop(Arc::from_raw(new));
}
}
}
inner
}
}
impl Drop for Event {
#[inline]
fn drop(&mut self) {
let inner: *mut Inner = *self.inner.get_mut();
// If the state pointer has been initialized, deallocate it.
if !inner.is_null() {
unsafe {
drop(Arc::from_raw(inner));
}
}
}
}
impl fmt::Debug for Event {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("Event { .. }")
}
}
impl Default for Event {
fn default() -> Event {
Event::new()
}
}
/// A guard waiting for a notification from an [`Event`].
///
/// There are two ways for a listener to wait for a notification:
///
/// 1. In an asynchronous manner using `.await`.
/// 2. In a blocking manner by calling [`EventListener::wait()`] on it.
///
/// If a notified listener is dropped without receiving a notification, dropping will notify
/// another active listener. Whether one *additional* listener will be notified depends on what
/// kind of notification was delivered.
pub struct EventListener {
/// A reference to [`Event`]'s inner state.
inner: Arc<Inner>,
/// A pointer to this listener's entry in the linked list.
entry: Option<NonNull<Entry>>,
}
unsafe impl Send for EventListener {}
unsafe impl Sync for EventListener {}
impl UnwindSafe for EventListener {}
impl RefUnwindSafe for EventListener {}
impl EventListener {
/// Blocks until a notification is received.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
/// let listener = event.listen();
///
/// // Notify `listener`.
/// event.notify(1);
///
/// // Receive the notification.
/// listener.wait();
/// ```
pub fn wait(self) {
self.wait_internal(None);
}
/// Blocks until a notification is received or a timeout is reached.
///
/// Returns `true` if a notification was received.
///
/// # Examples
///
/// ```
/// use std::time::Duration;
/// use event_listener::Event;
///
/// let event = Event::new();
/// let listener = event.listen();
///
/// // There are no notification so this times out.
/// assert!(!listener.wait_timeout(Duration::from_secs(1)));
/// ```
pub fn wait_timeout(self, timeout: Duration) -> bool {
self.wait_internal(Some(Instant::now() + timeout))
}
/// Blocks until a notification is received or a deadline is reached.
///
/// Returns `true` if a notification was received.
///
/// # Examples
///
/// ```
/// use std::time::{Duration, Instant};
/// use event_listener::Event;
///
/// let event = Event::new();
/// let listener = event.listen();
///
/// // There are no notification so this times out.
/// assert!(!listener.wait_deadline(Instant::now() + Duration::from_secs(1)));
/// ```
pub fn wait_deadline(self, deadline: Instant) -> bool {
self.wait_internal(Some(deadline))
}
/// Drops this listener and discards its notification (if any) without notifying another
/// active listener.
///
/// Returns `true` if a notification was discarded.
///
/// # Examples
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
/// let listener1 = event.listen();
/// let listener2 = event.listen();
///
/// event.notify(1);
///
/// assert!(listener1.discard());
/// assert!(!listener2.discard());
/// ```
pub fn discard(mut self) -> bool {
// If this listener has never picked up a notification...
if let Some(entry) = self.entry.take() {
let mut list = self.inner.lock();
// Remove the listener from the list and return `true` if it was notified.
if let State::Notified(_) = list.remove(entry, self.inner.cache_ptr()) {
return true;
}
}
false
}
/// Returns `true` if this listener listens to the given `Event`.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
/// let listener = event.listen();
///
/// assert!(listener.listens_to(&event));
/// ```
#[inline]
pub fn listens_to(&self, event: &Event) -> bool {
ptr::eq::<Inner>(&*self.inner, event.inner.load(Ordering::Acquire))
}
/// Returns `true` if both listeners listen to the same `Event`.
///
/// # Examples
///
/// ```
/// use event_listener::Event;
///
/// let event = Event::new();
/// let listener1 = event.listen();
/// let listener2 = event.listen();
///
/// assert!(listener1.same_event(&listener2));
/// ```
pub fn same_event(&self, other: &EventListener) -> bool {
ptr::eq::<Inner>(&*self.inner, &*other.inner)
}
fn wait_internal(mut self, deadline: Option<Instant>) -> bool {
// Take out the entry pointer and set it to `None`.
let entry = match self.entry.take() {
None => unreachable!("cannot wait twice on an `EventListener`"),
Some(entry) => entry,
};
// Set this listener's state to `Waiting`.
{
let mut list = self.inner.lock();
let e = unsafe { entry.as_ref() };
// Do a dummy replace operation in order to take out the state.
match e.state.replace(State::Notified(false)) {
State::Notified(_) => {
// If this listener has been notified, remove it from the list and return.
list.remove(entry, self.inner.cache_ptr());
return true;
}
// Otherwise, set the state to `Waiting`.
_ => e.state.set(State::Waiting(thread::current())),
}
}
// Wait until a notification is received or the timeout is reached.
loop {
match deadline {
None => thread::park(),
Some(deadline) => {
// Check for timeout.
let now = Instant::now();
if now >= deadline {
// Remove the entry and check if notified.
return self
.inner
.lock()
.remove(entry, self.inner.cache_ptr())
.is_notified();
}
// Park until the deadline.
thread::park_timeout(deadline - now);
}
}
let mut list = self.inner.lock();
let e = unsafe { entry.as_ref() };
// Do a dummy replace operation in order to take out the state.
match e.state.replace(State::Notified(false)) {
State::Notified(_) => {
// If this listener has been notified, remove it from the list and return.
list.remove(entry, self.inner.cache_ptr());
return true;
}
// Otherwise, set the state back to `Waiting`.
state => e.state.set(state),
}
}
}
}
impl fmt::Debug for EventListener {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("EventListener { .. }")
}
}
impl Future for EventListener {
type Output = ();
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut list = self.inner.lock();
let entry = match self.entry {
None => unreachable!("cannot poll a completed `EventListener` future"),
Some(entry) => entry,
};
let state = unsafe { &entry.as_ref().state };
// Do a dummy replace operation in order to take out the state.
match state.replace(State::Notified(false)) {
State::Notified(_) => {
// If this listener has been notified, remove it from the list and return.
list.remove(entry, self.inner.cache_ptr());
drop(list);
self.entry = None;
return Poll::Ready(());
}
State::Created => {
// If the listener was just created, put it in the `Polling` state.
state.set(State::Polling(cx.waker().clone()));
}
State::Polling(w) => {
// If the listener was in the `Polling` state, update the waker.
if w.will_wake(cx.waker()) {
state.set(State::Polling(w));
} else {
state.set(State::Polling(cx.waker().clone()));
}
}
State::Waiting(_) => {
unreachable!("cannot poll and wait on `EventListener` at the same time")
}
}
Poll::Pending
}
}
impl Drop for EventListener {
fn drop(&mut self) {
// If this listener has never picked up a notification...
if let Some(entry) = self.entry.take() {
let mut list = self.inner.lock();
// But if a notification was delivered to it...
if let State::Notified(additional) = list.remove(entry, self.inner.cache_ptr()) {
// Then pass it on to another active listener.
if additional {
list.notify_additional(1);
} else {
list.notify(1);
}
}
}
}
}
/// A guard holding the linked list locked.
struct ListGuard<'a> {
/// A reference to [`Event`]'s inner state.
inner: &'a Inner,
/// The actual guard that acquired the linked list.
guard: MutexGuard<'a, List>,
}
impl Drop for ListGuard<'_> {
#[inline]
fn drop(&mut self) {
let list = &mut **self;
// Update the atomic `notified` counter.
let notified = if list.notified < list.len {
list.notified
} else {
usize::MAX
};
self.inner.notified.store(notified, Ordering::Release);
}
}
impl Deref for ListGuard<'_> {
type Target = List;
#[inline]
fn deref(&self) -> &List {
&*self.guard
}
}
impl DerefMut for ListGuard<'_> {
#[inline]
fn deref_mut(&mut self) -> &mut List {
&mut *self.guard
}
}
/// The state of a listener.
enum State {
/// It has just been created.
Created,
/// It has received a notification.
///
/// The `bool` is `true` if this was an "additional" notification.
Notified(bool),
/// An async task is polling it.
Polling(Waker),
/// A thread is blocked on it.
Waiting(Thread),
}
impl State {
/// Returns `true` if this is the `Notified` state.
#[inline]
fn is_notified(&self) -> bool {
match self {
State::Notified(_) => true,
State::Created | State::Polling(_) | State::Waiting(_) => false,
}
}
}
/// An entry representing a registered listener.
struct Entry {
/// THe state of this listener.
state: Cell<State>,
/// Previous entry in the linked list.
prev: Cell<Option<NonNull<Entry>>>,
/// Next entry in the linked list.
next: Cell<Option<NonNull<Entry>>>,
}
/// A linked list of entries.
struct List {
/// First entry in the list.
head: Option<NonNull<Entry>>,
/// Last entry in the list.
tail: Option<NonNull<Entry>>,
/// The first unnotified entry in the list.
start: Option<NonNull<Entry>>,
/// Total number of entries in the list.
len: usize,
/// The number of notified entries in the list.
notified: usize,
/// Whether the cached entry is used.
cache_used: bool,
}
impl List {
/// Inserts a new entry into the list.
fn insert(&mut self, cache: NonNull<Entry>) -> NonNull<Entry> {
unsafe {
let entry = Entry {
state: Cell::new(State::Created),
prev: Cell::new(self.tail),
next: Cell::new(None),
};
let entry = if self.cache_used {
// Allocate an entry that is going to become the new tail.
NonNull::new_unchecked(Box::into_raw(Box::new(entry)))
} else {
// No need to allocate - we can use the cached entry.
self.cache_used = true;
cache.as_ptr().write(entry);
cache
};
// Replace the tail with the new entry.
match mem::replace(&mut self.tail, Some(entry)) {
None => self.head = Some(entry),
Some(t) => t.as_ref().next.set(Some(entry)),
}
// If there were no unnotified entries, this one is the first now.
if self.start.is_none() {
self.start = self.tail;
}
// Bump the entry count.
self.len += 1;
entry
}
}
/// Removes an entry from the list and returns its state.
fn remove(&mut self, entry: NonNull<Entry>, cache: NonNull<Entry>) -> State {
unsafe {
let prev = entry.as_ref().prev.get();
let next = entry.as_ref().next.get();
// Unlink from the previous entry.
match prev {
None => self.head = next,
Some(p) => p.as_ref().next.set(next),
}
// Unlink from the next entry.
match next {
None => self.tail = prev,
Some(n) => n.as_ref().prev.set(prev),
}
// If this was the first unnotified entry, move the pointer to the next one.
if self.start == Some(entry) {
self.start = next;
}
// Extract the state.
let state = if ptr::eq(entry.as_ptr(), cache.as_ptr()) {
// Free the cached entry.
self.cache_used = false;
entry.as_ref().state.replace(State::Created)
} else {
// Deallocate the entry.
Box::from_raw(entry.as_ptr()).state.into_inner()
};
// Update the counters.
if state.is_notified() {
self.notified -= 1;
}
self.len -= 1;
state
}
}
/// Notifies a number of entries.
#[cold]
fn notify(&mut self, mut n: usize) {
if n <= self.notified {
return;
}
n -= self.notified;
while n > 0 {
n -= 1;
// Notify the first unnotified entry.
match self.start {
None => break,
Some(e) => {
// Get the entry and move the pointer forward.
let e = unsafe { e.as_ref() };
self.start = e.next.get();
// Set the state of this entry to `Notified` and notify.
match e.state.replace(State::Notified(false)) {
State::Notified(_) => {}
State::Created => {}
State::Polling(w) => w.wake(),
State::Waiting(t) => t.unpark(),
}
// Update the counter.
self.notified += 1;
}
}
}
}
/// Notifies a number of additional entries.
#[cold]
fn notify_additional(&mut self, mut n: usize) {
while n > 0 {
n -= 1;
// Notify the first unnotified entry.
match self.start {
None => break,
Some(e) => {
// Get the entry and move the pointer forward.
let e = unsafe { e.as_ref() };
self.start = e.next.get();
// Set the state of this entry to `Notified` and notify.
match e.state.replace(State::Notified(true)) {
State::Notified(_) => {}
State::Created => {}
State::Polling(w) => w.wake(),
State::Waiting(t) => t.unpark(),
}
// Update the counter.
self.notified += 1;
}
}
}
}
}
/// Equivalent to `atomic::fence(Ordering::SeqCst)`, but in some cases faster.
#[inline]
fn full_fence() {
if cfg!(any(target_arch = "x86", target_arch = "x86_64")) {
// HACK(stjepang): On x86 architectures there are two different ways of executing
// a `SeqCst` fence.
//
// 1. `atomic::fence(SeqCst)`, which compiles into a `mfence` instruction.
// 2. `_.compare_exchange(_, _, SeqCst, SeqCst)`, which compiles into a `lock cmpxchg` instruction.
//
// Both instructions have the effect of a full barrier, but empirical benchmarks have shown
// that the second one is sometimes a bit faster.
//
// The ideal solution here would be to use inline assembly, but we're instead creating a
// temporary atomic variable and compare-and-exchanging its value. No sane compiler to
// x86 platforms is going to optimize this away.
let a = AtomicUsize::new(0);
let _ = a.compare_exchange(0, 1, Ordering::SeqCst, Ordering::SeqCst);
} else {
atomic::fence(Ordering::SeqCst);
}
}