sync: preserve permit state in notify_waiters

[simonlindholm]

Motivation

When there are no current waiters (state is EMPTY or NOTIFIED), Notify::notify_waiters just bumps the current generation counter and returns. But it does so non-atomically. Since the lock is held we know that the generation counter is untouched, and we cannot have entered state WAITING; we might however be overwriting NOTIFIED by EMPTY or EMPTY by NOTIFIED if we race with another thread.

The former case is bad: if notify_waiters() races with notify_one(), we might overwrite NOTIFIED by EMPTY and drop a permit.

The latter case is more or less fine, because if we race with a waiter that uses up a permit we can claim to have notified it and have its permit remain. But with enough effort one can show an API contract violation.

Solution

Use an atomic increment.

[zaharidichev]

Nice find, have you tried using loom to surface the problem

[simonlindholm]

Good point, that manages to reproduce the bug.

[Darksonn]

Thanks.