Re: [PATCH v2 3/4] test-ww_mutex: Handle transient -EDEADLK in test_cycle_work

[John Stultz]

On Mon, Jul 13, 2026 at 9:37 AM Håkon Bugge <haakon.bugge@xxxxxxxxxx> wrote:
>
> There is a timing issue in test_cycle_work(), in the sense that
> acquiring *a_mutex* after deadlock has been detected on the *b_mutex*,
> may not succeed immediately. This may lead to false negatives, which
> shows up in the log as:
>
>    cyclic deadlock not resolved, ret[77/93] = -35
>
> We re-factor the inner part test_cycle_work(), where we loop a few
> times attempting to acquire the mutexes in BAAB order. If the first A
> succeeds or the last B succeeds, we break out of the loop.
>
> Fixes: d1b42b800e5d ("locking/ww_mutex: Add kselftests for resolving ww_mutex cyclic deadlocks")
> Fixes: e4a02ed2aaf4 ("locking/ww_mutex: Fix runtime warning in the WW mutex selftest")
> Signed-off-by: Håkon Bugge <haakon.bugge@xxxxxxxxxx>
>

I've been running stress testing with this for a while as part of my
proxy-exec v31 submission, and I've not run into any issues so far.

This version definitely avoids my concern with the earlier one, so
thanks for the update.

One tiny nit below, but I wouldn't be upset if this went in unchanged.

Acked-by: John Stultz <jstultz@xxxxxxxxxx>

> ---
>
> v1 -> v2:
>       * Removed the end-less lock acquisition attempts, as that
>         strategy did not obey the big-rules of the WW
>         mutexes. Instead, re-factor the test, and loop a handful
>         times.
> ---
>  kernel/locking/test-ww_mutex.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/locking/test-ww_mutex.c b/kernel/locking/test-ww_mutex.c
> index 5ed91f2cf30d9..190b8ac5c5520 100644
> --- a/kernel/locking/test-ww_mutex.c
> +++ b/kernel/locking/test-ww_mutex.c
> @@ -294,6 +294,8 @@ static void test_cycle_work(struct work_struct *work)
>         struct test_cycle *cycle = container_of(work, typeof(*cycle), work);
>         struct ww_acquire_ctx ctx;
>         int err, erra = 0;
> +       const int max_attempts = 5;
> +       int attempt;
>
>         ww_acquire_init_noinject(&ctx, cycle->class);
>         ww_mutex_lock(&cycle->a_mutex, &ctx);
> @@ -302,13 +304,26 @@ static void test_cycle_work(struct work_struct *work)
>         wait_for_completion(&cycle->b_signal);
>
>         err = ww_mutex_lock(cycle->b_mutex, &ctx);
> -       if (err == -EDEADLK) {
> +       if (err != -EDEADLK)
> +               goto ok;

Nit: I'm not sure if "ok" is a great label for ww_mutex_lock returning
an error. "out" might be more intuitive?

thanks
-john