Re: Patch "sched: Do not call __put_task_struct() on rt if pi_blocked_on is set" has been added to the 6.16-stable tree

Re: Patch "sched: Do not call __put_task_struct() on rt if pi_blocked_on is set" has been added to the 6.16-stable tree

[Luis Claudio R. Goncalves]

On Fri, Aug 08, 2025 at 07:27:26PM -0400, Sasha Levin wrote:
> This is a note to let you know that I've just added the patch titled
> 
>     sched: Do not call __put_task_struct() on rt if pi_blocked_on is set
> 
> to the 6.16-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      sched-do-not-call-__put_task_struct-on-rt-if-pi_bloc.patch
> and it can be found in the queue-6.16 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.

Hi Sasha!

I am the author of that patch and I sent a follow-up patch that fixes a
mistake I made on the original patch:

    [RESEND PATCH] sched: restore the behavior of put_task_struct() for non-rt
    https://lore.kernel.org/all/aJOwe_ZS5rHXMrsO@uudg.org/

The patch you have does not match what is in the description. I unfortunately
sent the wrong version of the patch on the verge of leaving for a long vacation
and only noticed that when I returned. The code is correct, but does not
match the commit description and is a change that I would like to propose
later as an RFC, not the simpler bugfix originally intended.

I suggest waiting for the follow-up patch mentioned above to include both on
stable kernels.

Sorry for the inconvenience,
Luis

> 
> 
> commit 7bed29f5ad955444283dca82476dd92c59923f73
> Author: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
> Date:   Mon Jul 7 11:03:59 2025 -0300
> 
>     sched: Do not call __put_task_struct() on rt if pi_blocked_on is set
>     
>     [ Upstream commit 8671bad873ebeb082afcf7b4501395c374da6023 ]
>     
>     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.
>     
>     Fixes: 893cdaaa3977 ("sched: avoid false lockdep splat in put_task_struct()")
>     Suggested-by: Crystal Wood <crwood@redhat.com>
>     Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
>     Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>     Reviewed-by: Wander Lairson Costa <wander@redhat.com>
>     Reviewed-by: Valentin Schneider <vschneid@redhat.com>
>     Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>     Link: https://lore.kernel.org/r/aGvTz5VaPFyj0pBV@uudg.org
>     Signed-off-by: Sasha Levin <sashal@kernel.org>
> 
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index ca1db4b92c32..58ce71715268 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -135,24 +135,17 @@ static inline void put_task_struct(struct task_struct *t)
>  		return;
>  
>  	/*
> -	 * In !RT, it is always safe to call __put_task_struct().
> -	 * Under RT, we can only call it in preemptible context.
> -	 */
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
> -		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
> -
> -		lock_map_acquire_try(&put_task_map);
> -		__put_task_struct(t);
> -		lock_map_release(&put_task_map);
> -		return;
> -	}
> -
> -	/*
> -	 * under PREEMPT_RT, we can't call put_task_struct
> +	 * Under PREEMPT_RT, we can't call __put_task_struct
>  	 * in atomic context because it will indirectly
> -	 * acquire sleeping locks.
> +	 * acquire sleeping locks. The same is true if the
> +	 * current process has a mutex enqueued (blocked on
> +	 * a PI chain).
> +	 *
> +	 * In !RT, it is always safe to call __put_task_struct().
> +	 * Though, in order to simplify the code, resort to the
> +	 * deferred call too.
>  	 *
> -	 * call_rcu() will schedule delayed_put_task_struct_rcu()
> +	 * call_rcu() will schedule __put_task_struct_rcu_cb()
>  	 * to be called in process context.
>  	 *
>  	 * __put_task_struct() is called when
> @@ -165,7 +158,7 @@ static inline void put_task_struct(struct task_struct *t)
>  	 *
>  	 * delayed_free_task() also uses ->rcu, but it is only called
>  	 * when it fails to fork a process. Therefore, there is no
> -	 * way it can conflict with put_task_struct().
> +	 * way it can conflict with __put_task_struct().
>  	 */
>  	call_rcu(&t->rcu, __put_task_struct_rcu_cb);
>  }
> 
---end quoted text---

Re: Patch "sched: Do not call __put_task_struct() on rt if pi_blocked_on is set" has been added to the 6.16-stable tree

[Greg KH]

On Sun, Aug 10, 2025 at 10:20:22PM -0300, Luis Claudio R. Goncalves wrote:
> On Fri, Aug 08, 2025 at 07:27:26PM -0400, Sasha Levin wrote:
> > This is a note to let you know that I've just added the patch titled
> > 
> >     sched: Do not call __put_task_struct() on rt if pi_blocked_on is set
> > 
> > to the 6.16-stable tree which can be found at:
> >     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> > 
> > The filename of the patch is:
> >      sched-do-not-call-__put_task_struct-on-rt-if-pi_bloc.patch
> > and it can be found in the queue-6.16 subdirectory.
> > 
> > If you, or anyone else, feels it should not be added to the stable tree,
> > please let <stable@vger.kernel.org> know about it.
> 
> Hi Sasha!
> 
> I am the author of that patch and I sent a follow-up patch that fixes a
> mistake I made on the original patch:
> 
>     [RESEND PATCH] sched: restore the behavior of put_task_struct() for non-rt
>     https://lore.kernel.org/all/aJOwe_ZS5rHXMrsO@uudg.org/
> 
> The patch you have does not match what is in the description. I unfortunately
> sent the wrong version of the patch on the verge of leaving for a long vacation
> and only noticed that when I returned. The code is correct, but does not
> match the commit description and is a change that I would like to propose
> later as an RFC, not the simpler bugfix originally intended.
> 
> I suggest waiting for the follow-up patch mentioned above to include both on
> stable kernels.

Ok, I'll go drop this now but please let us know when both patches
shoudl be added.

thanks,

greg k-h