Re: [RFC PATCH 1/2] kernel/locking/ww_mutex: Add per-lock lock-check helpers

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 20, 2025 at 12:03:40PM +0100, Thomas Hellström wrote:
> Code using ww_mutexes typically by design have a number of
> such mutexes sharing the same ww_class, and within a ww transaction
> they are all lockdep annotated using a nest_lock which means
> that multiple ww_mutexes of the same lockdep class may be locked at
> the same time. That means that lock_is_held() returns true and
> lockdep_assert_held() doesn't fire as long as there is a *single*
> ww_mutex held of the same class. IOW within a WW transaction.
> 
> Code using these mutexes typically want to assert that individual
> ww_mutexes are held. Not that any ww_mutex of the same class is
> held.
> 
> Introduce functions that can be used for that.
> 
> RFC: Placement of the functions? lockdep.c? Are the #ifdefs testing for
> the correct config?

Yeah, I think so.

Ack on this.

> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>  include/linux/ww_mutex.h | 18 ++++++++++++++++++
>  kernel/locking/mutex.c   | 10 ++++++++++
>  2 files changed, 28 insertions(+)
> 
> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> index 45ff6f7a872b..7bc0f533dea6 100644
> --- a/include/linux/ww_mutex.h
> +++ b/include/linux/ww_mutex.h
> @@ -380,4 +380,22 @@ static inline bool ww_mutex_is_locked(struct ww_mutex *lock)
>  	return ww_mutex_base_is_locked(&lock->base);
>  }
>  
> +#ifdef CONFIG_PROVE_LOCKING
> +
> +bool ww_mutex_held(struct ww_mutex *lock);
> +
> +#else /* CONFIG_PROVE_LOCKING */
> +
> +static inline bool ww_mutex_held(struct ww_mutex *lock)
> +{
> +	return true;
> +}
> +
> +#endif /* CONFIG_PROVE_LOCKING */
> +
> +static inline void ww_mutex_assert_held(struct ww_mutex *lock)
> +{
> +	lockdep_assert(ww_mutex_held(lock));
> +}
> +
>  #endif
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index de7d6702cd96..37868b739efd 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -1174,3 +1174,13 @@ int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
>  	return 1;
>  }
>  EXPORT_SYMBOL(atomic_dec_and_mutex_lock);
> +
> +#ifdef CONFIG_PROVE_LOCKING
> +
> +bool ww_mutex_held(struct ww_mutex *lock)
> +{
> +	return __ww_mutex_owner(&lock->base) == current;
> +}
> +EXPORT_SYMBOL(ww_mutex_held);
> +
> +#endif /* CONFIG_PROVE_LOCKING */
> -- 
> 2.51.1
>