[External] : Re: [POTENTIAL BUG] Potential FIFO violation in BlockingQueue under high contention and suggestion for fair mode in ArrayBlockingQueue and LinkedBlockingQueue

[Archie Cobbs]

Apologies in advance if I'm misunderstanding anything...

On Thu, Sep 5, 2024 at 2:05 PM Viktor Klang <viktor.klang at oracle.com>
wrote:

>  Thread state polling aside, for as long as Condition::await() is allowed
> to spuriously wake, FIFO just cannot be "guaranteed".


What about this statement in the Javadoc for ReentrantLock.newCondition():

The ordering of lock reacquisition for threads returning from waiting
> methods is the same as for threads initially acquiring the lock, which is
> in the default case not specified, but for *fair* locks favors those
> threads that have been waiting the longest.


So what you're saying is that a spurious wakeup on a Condition is not the
same thing as a spurious signal() on a Condition; if it were, then the
above statement would apply and FIFO ordering would be preserved.

Of course, a spurious wakeup would not find the condition being waited on
satisfied unless there was a big coincidence. So an ordering violation that
actually mattered should be exceedingly rare.

Anyway, this does seem to be a glitch in how things are supposed to work.
That is: there can be no guaranteed ordering for Condition waiters when
there can be spurious wakeups.

Maybe this corner case should be documented. Or better yet, fix the bug by
requiring Condition to "filter out" spurious wakeups if preserving FIFO
ordering (it should be possible).

-Archie

-- 
Archie L. Cobbs
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