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Drop wakers after unlocking the mutex in Notify #5471

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Feb 19, 2023
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23 changes: 19 additions & 4 deletions tokio/src/sync/notify.rs
Original file line number Diff line number Diff line change
Expand Up @@ -917,10 +917,14 @@ impl Notified<'_> {
}
}

let mut old_waker = None;
if waker.is_some() {
// Safety: called while locked.
//
// The use of `old_waiter` here is not necessary, as the field is always
// None when we reach this line.
unsafe {
(*waiter.get()).waker = waker;
old_waker = std::mem::replace(&mut (*waiter.get()).waker, waker);
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old_waker is always None here, as this is the first place we store a waker and from here we always transition to Waiting with no return.

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I am reluctant to rely on assumptions like "waker is always None here" or "waker is always Some here", and prefer to program defensively. We've run into trouble with those in the past due to this type of assumption being broken by future changes to the code.

Anyway, I added a comment to note this.

}
}

Expand All @@ -931,6 +935,9 @@ impl Notified<'_> {

*state = Waiting;

drop(waiters);
drop(old_waker);

return Poll::Pending;
}
Waiting => {
Expand All @@ -945,12 +952,13 @@ impl Notified<'_> {

// Safety: called while locked
let w = unsafe { &mut *waiter.get() };
let mut old_waker = None;

if w.notified.is_some() {
// Our waker has been notified and our waiter is already removed from
// the list. Reset the notification and convert to `Done`.
old_waker = std::mem::take(&mut w.waker);
w.notified = None;
w.waker = None;
*state = Done;
} else if get_num_notify_waiters_calls(curr) != *notify_waiters_calls {
// Before we add a waiter to the list we check if these numbers are
Expand All @@ -960,7 +968,7 @@ impl Notified<'_> {
// We can treat the waiter as notified and remove it from the list, as
// it would have been notified in the `notify_waiters` call anyways.

w.waker = None;
old_waker = std::mem::take(&mut w.waker);

// Safety: we hold the lock, so we have an exclusive access to the list.
// The list is used in `notify_waiters`, so it must be guarded.
Expand All @@ -975,10 +983,14 @@ impl Notified<'_> {
None => true,
};
if should_update {
w.waker = Some(waker.clone());
old_waker = std::mem::replace(&mut w.waker, Some(waker.clone()));
}
}

// Drop the old waker after releasing the lock.
drop(waiters);
drop(old_waker);

return Poll::Pending;
}

Expand All @@ -988,6 +1000,9 @@ impl Notified<'_> {
// is helpful to visualize the scope of the critical
// section.
drop(waiters);

// Drop the old waker after releasing the lock.
drop(old_waker);
}
Done => {
return Poll::Ready(());
Expand Down