Re: [PATCH v6] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

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

On 2025-07-07 11:03:59 [-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);
> 
> 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@redhat.com>
> Reviewed-by: Wander Lairson Costa <wander@redhat.com>
> Fixes: 893cdaaa3977 ("sched: avoid false lockdep splat in put_task_struct()")
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Side note: This simplifies the call chain to always free the task struct
via RCU. This also means that the stack is not immediately available for
recycle (CONFIG_VMAP_STACK) but after the grace period. Based on my
testing the new task was also deployed on a remote CPU therefore I
wouldn't worry too much here. Just wanted to point the only difference I
could come up with.

Sebastian