Re: [RESEND PATCH] sched: restore the behavior of put_task_struct() for non-rt

[Sebastian Andrzej Siewior]

On 2025-08-06 16:43:55 [-0300], Luis Claudio R. Goncalves wrote:
> Commit 8671bad873eb ("sched: Do not call __put_task_struct() on rt
> if pi_blocked_on is set") changed the behavior of put_task_struct()
> unconditionally, even when PREEMPT_RT was not enabled, in clear mismatch
> with the commit description.
> 
> Restore the previous behavior of put_task_struct() for the PREEMPT_RT
> disabled case.
> 
> Fixes: 8671bad873eb ("sched: Do not call __put_task_struct() on rt if pi_blocked_on is set")
> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
> ---
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index ea41795a352b..51678a541477 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -130,6 +133,16 @@ 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(). */

I don't want to drag this but this comment is obvious for anyone who is
fluent in C. It is just a statement with no explanation.
An important note would be that the atomic context restriction only
apply to PREEMPT_RT and therefore we have this context override for
lockdep below. The other question would be why don't we do this
unconditionally regardless of PREEMPT_RT. The only reason I could find
is that releasing the task here from the "exit path" makes the vmap
stack "earlier" available for reuse.

> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		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
>  	 * in atomic context because it will indirectly
> @@ -137,10 +150,6 @@ static inline void put_task_struct(struct task_struct *t)
>  	 * 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 __put_task_struct_rcu_cb()
>  	 * to be called in process context.
>  	 *
> 

Sebastian