Struct polkadot_node_subsystem::gen::futures::task::AtomicWaker
source · pub struct AtomicWaker { /* private fields */ }
Expand description
A synchronization primitive for task wakeup.
Sometimes the task interested in a given event will change over time.
An AtomicWaker
can coordinate concurrent notifications with the consumer
potentially “updating” the underlying task to wake up. 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 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
.
Memory ordering
Calling register
“acquires” all memory “released” by calls to wake
before the call to register
. Later calls to wake
will wake the
registered waker (on contention this wake might be triggered in register
).
For concurrent calls to register
(should be avoided) the ordering is only
guaranteed for the winning call.
Examples
Here is a simple example providing a Flag
that can be signalled manually
when it is ready.
use futures::future::Future;
use futures::task::{Context, Poll, AtomicWaker};
use std::sync::Arc;
use std::sync::atomic::AtomicBool;
use std::sync::atomic::Ordering::Relaxed;
use std::pin::Pin;
struct Inner {
waker: AtomicWaker,
set: AtomicBool,
}
#[derive(Clone)]
struct Flag(Arc<Inner>);
impl Flag {
pub fn new() -> Self {
Self(Arc::new(Inner {
waker: AtomicWaker::new(),
set: AtomicBool::new(false),
}))
}
pub fn signal(&self) {
self.0.set.store(true, Relaxed);
self.0.waker.wake();
}
}
impl Future for Flag {
type Output = ();
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
// quick check to avoid registration if already done.
if self.0.set.load(Relaxed) {
return Poll::Ready(());
}
self.0.waker.register(cx.waker());
// Need to check condition **after** `register` to avoid a race
// condition that would result in lost notifications.
if self.0.set.load(Relaxed) {
Poll::Ready(())
} else {
Poll::Pending
}
}
}
Implementations§
source§impl AtomicWaker
impl AtomicWaker
sourcepub const fn new() -> AtomicWaker
pub const fn new() -> AtomicWaker
Create an AtomicWaker
.
sourcepub fn register(&self, waker: &Waker)
pub fn register(&self, waker: &Waker)
Registers the waker to be notified on calls to wake
.
The new task will take place of any previous tasks 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
notify the register
caller’s task and deregister the waker from future
notifications. Because of this, callers should ensure register
gets
invoked with a new Waker
each time they require a wakeup.
It is safe to call register
with multiple other threads concurrently
calling wake
. 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.
Examples
Here is how register
is used when implementing a flag.
use futures::future::Future;
use futures::task::{Context, Poll, AtomicWaker};
use std::sync::atomic::AtomicBool;
use std::sync::atomic::Ordering::Relaxed;
use std::pin::Pin;
struct Flag {
waker: AtomicWaker,
set: AtomicBool,
}
impl Future for Flag {
type Output = ();
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
// Register **before** checking `set` to avoid a race condition
// that would result in lost notifications.
self.waker.register(cx.waker());
if self.set.load(Relaxed) {
Poll::Ready(())
} else {
Poll::Pending
}
}
}
sourcepub fn wake(&self)
pub fn wake(&self)
Calls wake
on the last Waker
passed to register
.
If register
has not been called yet, then this does nothing.
sourcepub fn take(&self) -> Option<Waker>
pub fn take(&self) -> Option<Waker>
Returns the last Waker
passed to register
, so that the user can wake it.
Sometimes, just waking the AtomicWaker is not fine grained enough. This allows the user to take the waker and then wake it separately, rather than performing both steps in one atomic action.
If a waker has not been registered, this returns None
.
Trait Implementations§
source§impl Debug for AtomicWaker
impl Debug for AtomicWaker
source§impl Default for AtomicWaker
impl Default for AtomicWaker
source§fn default() -> AtomicWaker
fn default() -> AtomicWaker
impl Send for AtomicWaker
impl Sync for AtomicWaker
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