Re: [PATCH v16 2/7] locking/mutex: Rework task_struct::blocked_on

[Juri Lelli]

Hi John,

On 11/04/25 23:02, John Stultz wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 
> Track the blocked-on relation for mutexes, to allow following this
> relation at schedule time.
> 
>    task
>      | blocked-on
>      v
>    mutex
>      | owner
>      v
>    task
> 
> This all will be used for tracking blocked-task/mutex chains
> with the prox-execution patch in a similar fashion to how
> priority inheritance is done with rt_mutexes.
> 
> For serialization, blocked-on is only set by the task itself
> (current). And both when setting or clearing (potentially by
> others), is done while holding the mutex::wait_lock.
> 
> Cc: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> Cc: Qais Yousef <qyousef@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Metin Kaya <Metin.Kaya@xxxxxxx>
> Cc: Xuewen Yan <xuewen.yan94@xxxxxxxxx>
> Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Cc: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> Cc: kernel-team@xxxxxxxxxxx
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> [minor changes while rebasing]
> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Connor O'Brien <connoro@xxxxxxxxxx>
> [jstultz: Fix blocked_on tracking in __mutex_lock_common in error paths]
> Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
> ---

...

> @@ -940,6 +954,14 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
>  		next = waiter->task;
>  
>  		debug_mutex_wake_waiter(lock, waiter);
> +		/*
> +		 * Unlock wakeups can be happening in parallel
> +		 * (when optimistic spinners steal and release
> +		 * the lock), so blocked_on may already be
> +		 * cleared here.
> +		 */
> +		WARN_ON(next->blocked_on && next->blocked_on != lock);
> +		next->blocked_on = NULL;

Here and below, why the WARN_ON() if the fact that blocked_on has been
cleared already it's an OK situation? Ah, maybe it's catching the more
worrying situation that the lock has changed since the task blocked?

>  		wake_q_add(&wake_q, next);
>  	}
>  
> diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
> index 37f025a096c9d..00db40946328e 100644
> --- a/kernel/locking/ww_mutex.h
> +++ b/kernel/locking/ww_mutex.h
> @@ -284,6 +284,14 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
>  #ifndef WW_RT
>  		debug_mutex_wake_waiter(lock, waiter);
>  #endif
> +		/*
> +		 * When waking up the task to die, be sure to clear the
> +		 * blocked_on pointer. Otherwise we can see circular
> +		 * blocked_on relationships that can't resolve.
> +		 */
> +		WARN_ON(waiter->task->blocked_on &&
> +			waiter->task->blocked_on != lock);
> +		waiter->task->blocked_on = NULL;
>  		wake_q_add(wake_q, waiter->task);
>  	}

Thanks,
Juri