* 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
[not found] <20250808232726.1423484-1-sashal@kernel.org>
@ 2025-08-11 1:20 ` Luis Claudio R. Goncalves
2025-08-12 8:55 ` Greg KH
0 siblings, 1 reply; 2+ messages in thread
From: Luis Claudio R. Goncalves @ 2025-08-11 1:20 UTC (permalink / raw)
To: Sasha Levin, stable, stable-commits, Sebastian Andrzej Siewior,
Clark Williams, Steven Rostedt
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---
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