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

[Peter Zijlstra <peterz@infradead.org>]

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?

> Suggested-by: Crystal Wood <crwood@redhat.com>
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
> ---
> 
> Resent as a gentle reminder, because this issue results in scary backtraces,
> not obvious to debug and pinpoint root cause.
> 
> v2: (Rostedt) remove the #ifdef from put_task_struct() and create
>     tsk_is_pi_blocked_on() in sched.h to make the change cleaner.
> v3: (Sebastian, PeterZ) always call the deferred __put_task_struct() on RT.
> v4: Fix the implementation of what was requested on v3.
> 
>  include/linux/sched/task.h |   17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 0f2aeb37bbb04..51678a541477a 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -134,11 +134,8 @@ static inline void put_task_struct(struct task_struct *t)
>  	if (!refcount_dec_and_test(&t->usage))
>  		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()) {
> +	/* In !RT, it is always safe to call __put_task_struct(). */
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
>  		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
>  
>  		lock_map_acquire_try(&put_task_map);
> @@ -148,11 +145,13 @@ static inline void put_task_struct(struct task_struct *t)
>  	}
>  
>  	/*
> -	 * 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).
>  	 *
> -	 * 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 +164,7 @@ 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 forwarded message -----
>