Re: [PATCH] selftests/futex: Retry the wake in futex_wait instead of sleeping

[Mathieu Desnoyers]

On 2026-11-08 09:23, Mathieu Desnoyers wrote:
> Each futex_wait test starts a waiter thread, which calls futex_wait()
> with a 30ms timeout. The main thread sleeps 10ms, then expects
> futex_wake() to wake one waiter. So the test assumes that the waiter
> reaches futex_wait() within 10ms, and that the main thread runs again
> within 30ms.
>
> A busy machine breaks both. The built-in locktorture at times holds a
> spinlock for 100ms with preemption off. With it, private_futex failed in
> about 0.5% of the runs in 8-CPU VMs, on v7.4-rc1.
> Timestamps showed two cases. The waiter started up to 75ms late, after
> the wake. Or the main thread woke up to 105ms late, after the waiter
> had timed out.
>
> Fix this by retrying futex_wake() every millisecond until it wakes the
> waiter, for up to 5s. Give the waiter a timeout of 10s, so that it
> outlasts the retries. The tests then wait for the waiter, instead of
> assuming when it runs.
>
> With this patch, private_futex passed 6000 runs of 6000 in the same VMs,
> and the three tests 2000 runs of 2000.
Self-nack. That's flaky as well. A bounded retry can still fail. This
needs a proper synchronization flag. Will respin a v2.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com