Re: [PATCH v5] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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@redhat.com>
> Fixes: 893cdaaa3977 ("sched: avoid false lockdep splat in put_task_struct()")
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>

Sebastian