Re: [PATCH 1/8] perf bench futex: Retry futex_wait() when interrupted by a signal

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] perf: deadlock in futex wake-parallel due to missed one-shot futex_wake()
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commit 4425d175409b64dbe77117f655f0ffa4eca2b864
Author: Michal Pluta <michalpl2003@gmail.com>

perf bench futex: Retry futex_wait() when interrupted by a signal

The worker threads in 'futex wake' and 'futex wake-parallel' are meant
to go back to sleep when a signal interrupts futex_wait(), but the check
never works. futex_wait() returns -1 and sets errno, so comparing its
return value with EINTR is always true and the loop ends at the first
interruption.

> diff --git a/tools/perf/bench/futex-wake-parallel.c b/tools/perf/bench/futex-wake-parallel.c
> index 6aede7c46b337..a089d8ee4b023 100644
> --- a/tools/perf/bench/futex-wake-parallel.c
> +++ b/tools/perf/bench/futex-wake-parallel.c
> @@ -139,10 +139,9 @@ static void *blocked_workerfn(void *arg __maybe_unused)
>  	cond_wait(&thread_worker, &thread_lock);
>  	mutex_unlock(&thread_lock);
>  
> -	while (1) { /* handle spurious wakeups */
> -		if (futex_wait(&futex, 0, NULL, futex_flag) != EINTR)
> -			break;
> -	}
> +	/* Retry if the wait was interrupted by a signal. */
> +	while (futex_wait(&futex, 0, NULL, futex_flag) == -1 && errno == EINTR)
> +		;

[Severity: Medium]
Can this retry loop cause a deadlock if a signal interrupts the wait?

If blocked_workerfn() receives a signal, futex_wait() returns -1 and
errno == EINTR. The thread then temporarily leaves the futex wait queue before
looping back to call futex_wait() again.

If waking_workerfn() in tools/perf/bench/futex-wake-parallel.c happens to
execute its single futex_wake() call during this unqueued window:

tools/perf/bench/futex-wake-parallel.c:waking_workerfn() {
    ...
    waker->nwoken = futex_wake(&futex, nwakes, futex_flag);
    if (waker->nwoken != nwakes)
        warnx("couldn't wakeup all tasks (%d/%d)",
    ...
}

Will the waker thread miss the unqueued worker and exit, leaving the worker to
block forever on its next futex_wait() and causing the main thread to hang in
pthread_join()?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260926190407.1106421-1-michalpl2003@gmail.com?part=1