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

[Oleg Nesterov <oleg@redhat.com>]

On 07/07, Luis Claudio R. Goncalves wrote:
>
> 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.
                              ^^^^^^^^^^^^^
Confused... with this patch put_task_struct() always uses the deferred
call, regardless of PREEMPT_RT?

Oleg.

> --- 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);
>  }
> -- 
> 2.50.0
>