Re: [RESEND PATCH v4] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

[Wander Lairson Costa]

On Tue, Jun 17, 2025 at 11:36:27AM +0200, Sebastian Andrzej Siewior wrote:
> On 2025-06-17 11:26:09 [+0200], Peter Zijlstra wrote:
> > On Fri, Jun 13, 2025 at 12:05:14PM -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);
> > > 
> > > Fix that by unconditionally resorting to the deferred call to
> > > __put_task_struct() if PREEMPT_RT is enabled.
> > > 
> > 
> > Should this have a Fixes: tag and go into /urgent?
> 
> I would say so. I'm not sure what caused it. I think Luis said at some
> point that it is caused by a sched_ext case or I mixed it up with
> something. Luis?
> 
> The other question I have, do we need to distinguish between PREEMPT_RT
> and not or can we do this unconditionally?
> 

That's something I had been wondering myself. However, since this code
runs in multiple places, I was concerned it might trigger some obscure
corner-case issue. In any case, if we decide to remove the
PREEMPT_RT conditional, I’d prefer to handle that in a follow-up patch.

> Sebastian
>