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

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Message-ID: <d2e98b4d7f76534c68c1a0cc30fdd61b109cbd3c.camel@redhat.com>
Date: Wed, 25 Jun 2025 16:48:35 -0500
From: Crystal Wood <crwood@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, "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>,
 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 Wed, 2025-06-18 at 09:03 +0200, Sebastian Andrzej Siewior wrote:
> 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.

sched_ext_free() just happens to be the first cleanup function called,
so that's where the blowup happens.  I think the "nobody managed to
trigger it" was because we didn't have the pi_blocked_on assert until
recently -- and my "other cases with a similar cause" was probably older
kernels with the assert backported, but not sched_ext, so the backtrace
was different.

-Crystal