Re: [PATCH v3] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

[Peter Zijlstra]

On Thu, Apr 10, 2025 at 09:10:12AM -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().
> 
> 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 and PeterZ) always call the RCU deferred __put_task_struct().

Changelog goes below the --- line.

> Suggested-by: Crystal Wood <crwood@xxxxxxxxxx>
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
> ---
>  include/linux/sched/task.h |   20 +++++---------------
>  1 file changed, 5 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 0f2aeb37bbb04..49847efe5559e 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -134,22 +134,12 @@ 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;
> -	}

I don't think you've substantiated why the !PREEMPT_RT case needs to go.

> -
> -	/*
> -	 * under PREEMPT_RT, we can't call put_task_struct
> +	 * In !RT, it is always safe to call __put_task_struct(),
> +	 * but 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()
>  	 * to be called in process context.
> -- 
> 2.49.0
>