linux-kernel - Re: [PATCH v6] sched: do not call __put_task_struct() on rt if pi_blocked

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Message-ID: <xhsmhtt3k10jh.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Thu, 10 Jul 2025 18:19:14 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>, Peter Zijlstra
 <peterz@...radead.org>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 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>, Vincent Guittot <vincent.guittot@...aro.org>, Thomas
 Gleixner <tglx@...utronix.de>, Wander Lairson Costa <wander@...hat.com>,
 lclaudio00@...il.com
Subject: Re: [PATCH v6] sched: do not call __put_task_struct() on rt if
 pi_blocked_on is set

On 07/07/25 11:03, 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);
>
> This is not a SCHED_EXT bug. The first cleanup function called by
> __put_task_struct() is sched_ext_free() and it happens to take a
> (RT) spin_lock, which in the scenario described above, would trigger
> the lockdep assertion of "!current->pi_blocked_on".
>
> Crystal Wood was able to identify the problem as __put_task_struct()
> being called during rt_mutex_adjust_prio_chain(), in the context of
> a process with a mutex enqueued.
>
> Instead of adding more complex conditions to decide when to directly
> call __put_task_struct() and when to defer the call, unconditionally
> resort to the deferred call on PREEMPT_RT to simplify the code.
>
> Suggested-by: Crystal Wood <crwood@...hat.com>
> Reviewed-by: Wander Lairson Costa <wander@...hat.com>
> Fixes: 893cdaaa3977 ("sched: avoid false lockdep splat in put_task_struct()")
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@...hat.com>

FWIW:

Reviewed-by: Valentin Schneider <vschneid@...hat.com>