linux-kernel - Re: [PATCH v16 3/7] locking/mutex: Add p->blocked

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Message-ID: <Z_zQvuPOlMj3oM3z@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 14 Apr 2025 11:09:18 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Valentin Schneider <valentin.schneider@....com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Metin Kaya <Metin.Kaya@....com>,
	Xuewen Yan <xuewen.yan94@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Suleiman Souhlal <suleiman@...gle.com>, kernel-team@...roid.com,
	Connor O'Brien <connoro@...gle.com>
Subject: Re: [PATCH v16 3/7] locking/mutex: Add p->blocked_on wrappers for
 correctness checks

Hi John,

On 11/04/25 23:02, John Stultz wrote:
> From: Valentin Schneider <valentin.schneider@....com>
> 
> 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@...dia.com>
> Cc: Qais Yousef <qyousef@...alina.io>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Metin Kaya <Metin.Kaya@....com>
> Cc: Xuewen Yan <xuewen.yan94@...il.com>
> Cc: K Prateek Nayak <kprateek.nayak@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Suleiman Souhlal <suleiman@...gle.com>
> Cc: kernel-team@...roid.com
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> [fix conflicts, call in more places]
> Signed-off-by: Connor O'Brien <connoro@...gle.com>
> [jstultz: tweaked commit subject, reworked a good bit]
> Signed-off-by: John Stultz <jstultz@...gle.com>
> ---

...

> +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