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

[Crystal Wood <crwood@redhat.com>]

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