[RFC][PATCH] locking: Fix __clear_task_blocked_on() warning from __ww_mutex_wound() path

[John Stultz]

On Thu, Jul 31, 2025 at 10:09 PM K Prateek Nayak <kprateek.nayak at amd.com> wrote:
> At the very least I think we should make a local copy of "p->blocked_on"
> to see a consistent view throughout __clear_task_blocked_on() - task either
> sees it is blocked on the mutex and clear "p->blocked_on", or it sees it is
> blocked on nothing and still clears "p->blocked_on".
>
> (Tested lightly with syzbot's C reproducer)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 02c340450469..f35d93cca64f 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2165,6 +2165,8 @@ static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m)
>  static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m)
>  {
>         if (m) {
> +               struct mutex *blocked_on = p->blocked_on;
> +
>                 /* Currently we serialize blocked_on under the mutex::wait_lock */
>                 lockdep_assert_held_once(&m->wait_lock);
>                 /*
> @@ -2172,7 +2174,7 @@ static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *
>                  * blocked_on relationships, but make sure we are not
>                  * clearing the relationship with a different lock.
>                  */
> -               WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m);
> +               WARN_ON_ONCE(m && blocked_on && blocked_on != m);
>         }
>         p->blocked_on = NULL;
>  }
> ---
>
> End result is the same, only that we avoid an unnecessary splat in this
> very unlikely case and save ourselves some head scratching later :)

Good point. Thanks for suggesting this! I'll rework to include both
this and Maarten's suggestions.

Thank you for the feedback!
-john