Re: [PATCH v16 3/7] locking/mutex: Add p->blocked_on wrappers for correctness checks

[Juri Lelli]

Hi John,

On 11/04/25 23:02, John Stultz wrote:
> From: Valentin Schneider <valentin.schneider@xxxxxxx>
> 
> This lets us assert mutex::wait_lock is held whenever we access
> p->blocked_on, as well as warn us for unexpected state changes.
> 
> 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: Valentin Schneider <valentin.schneider@xxxxxxx>
> [fix conflicts, call in more places]
> Signed-off-by: Connor O'Brien <connoro@xxxxxxxxxx>
> [jstultz: tweaked commit subject, reworked a good bit]
> Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
> ---

...

> +static inline struct mutex *__get_task_blocked_on(struct task_struct *p)
> +{
> +	return READ_ONCE(p->blocked_on);
> +}
> +

Shouldn't we check that wait_lock is held also when reading blocked_on?
And, if that's the case, why do we use READ_ONCE?

Thanks,
Juri