Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe function

[Oleg Nesterov <oleg@redhat.com>]

On 01/20, Wander Lairson Costa wrote:
>
> +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		/*
> +		 * Decrement the refcount explicitly to avoid unnecessarily
> +		 * calling call_rcu.
> +		 */
> +		if (refcount_dec_and_test(&task->usage))
> +			/*
> +			 * under PREEMPT_RT, we can't call put_task_struct
> +			 * in atomic context because it will indirectly
> +			 * acquire sleeping locks.
> +			 */
> +			call_rcu(&task->rcu, __delayed_put_task_struct);
                                  ^^^^^^^^^
I am not sure the usage of task->rcu is safe...

Suppose that, before __delayed_put_task_struct() is called by RCU, this task
does the last schedule and calls put_task_struct_rcu_user().

And, can't we simply turn put_task_struct() into something like

	put_task_struct(struct task_struct *t)
	{
		if (refcount_dec_and_test(&t->usage)) {
			if (IS_ENABLED(CONFIG_PREEMPT_RT)
			    && (in_atomic() || irqs_disabled()))
				call_rcu(...);
			else
				__put_task_struct(t);
		}
	}

?

Oleg.