linux-kernel - Re: [PATCH v5] sched: do not call __put_task_struct() on rt if pi_blocked

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Message-ID: <20250618070350.19JbjFnG@linutronix.de>
Date: Wed, 18 Jun 2025 09:03:50 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
	David Vernet <dvernet@...a.com>, Barret Rhoden <brho@...gle.com>,
	Josh Don <joshdon@...gle.com>, Crystal Wood <crwood@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Juri Lelli <juri.lelli@...hat.com>, Ben Segall <bsegall@...gle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wander Lairson Costa <wander@...hat.com>
Subject: Re: [PATCH v5] sched: do not call __put_task_struct() on rt if
 pi_blocked_on is set

On 2025-06-17 11:00:36 [-0300], Luis Claudio R. Goncalves wrote:
> With PREEMPT_RT enabled, some of the calls to put_task_struct() coming
> from rt_mutex_adjust_prio_chain() could happen in preemptible context and
> with a mutex enqueued. That could lead to this sequence:
> 
>         rt_mutex_adjust_prio_chain()
>           put_task_struct()
>             __put_task_struct()
>               sched_ext_free()
>                 spin_lock_irqsave()
>                   rtlock_lock() --->  TRIGGERS
>                                       lockdep_assert(!current->pi_blocked_on);

Maybe with the addition of

| The first case was observed with sched_ext_free(). 
| Crystal Wood was able to reproduce the problem to __put_task_struct()
| being called during rt_mutex_adjust_prio_chain().

The first sentence will imply a Fixes: with the introduction of
sched_ext. The second implies that the original fix was not complete and
nobody managed to trigger it until now.

> Fix that by unconditionally resorting to the deferred call to
> __put_task_struct() if PREEMPT_RT is enabled.
> 
> Suggested-by: Crystal Wood <crwood@...hat.com>
> Fixes: 893cdaaa3977 ("sched: avoid false lockdep splat in put_task_struct()")
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@...hat.com>

Sebastian