Re: [PATCH] locking/mutex: Add debug code to help catching violation of mutex lifetime rule

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Jul 09, 2025 at 03:39:10PM -0400, Waiman Long wrote:
> Callers of mutex_lock/unlock() must ensure the validity of the memory
> objects holding mutexes until after the return of all outstanding
> and upcoming mutex_unlock() calls. Add a cond_resched() call in
> __mutex_unlock_slowpath() to help KASAN catch a violation of this
> rule. This will enforce a context switch if another runnable task is
> present and the CPU is not in an atomic context.
> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Waiman Long <longman@redhat.com>

Meta question: are we able to construct a case that shows this can help
detect the issue?

Regards,
Boqun

> ---
>  kernel/locking/mutex.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index a39ecccbd106..dd107fb9dad7 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -931,6 +931,17 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
>  		}
>  	}
>  
> +#if defined(CONFIG_KASAN) && defined(CONFIG_DEBUG_MUTEXES)
> +	/*
> +	 * Mutex users must ensure that the memory object holding the mutex
> +	 * remains valid until after the return of all the outstanding and
> +	 * upcoming mutex_unlock() calls. Enforce a context switch here if
> +	 * another runnable task is present and this CPU is not in an atomic
> +	 * context to increase the chance that KASAN can catch a violation of
> +	 * this rule.
> +	 */
> +	cond_resched();
> +#endif
>  	raw_spin_lock_irqsave(&lock->wait_lock, flags);
>  	debug_mutex_unlock(lock);
>  	if (!list_empty(&lock->wait_list)) {
> -- 
> 2.50.0
>