Re: [PATCH RFC tip/core/rcu 8/9] rcu: Make RCU-tasks track exiting tasks

[Oleg Nesterov <oleg@redhat.com>]

On 07/28, Paul E. McKenney wrote:
>
> This commit adds synchronization with exiting tasks, so that RCU-tasks
> avoids waiting on tasks that no longer exist.

I don't understand this patch yet, but it seems that it adds more than
just synchronization with exiting tasks?

> +		ACCESS_ONCE(t->rcu_tasks_holdout) = 1;
> +		spin_unlock(&t->rcu_tasks_lock);
> +		smp_mb();  /* Order ->rcu_tasks_holdout store before "if". */
> +		if (t == current || !ACCESS_ONCE(t->on_rq) || is_idle_task(t)) {
> +			smp_store_release(&t->rcu_tasks_holdout, 0);
> +			goto next_thread;
> +		}

This should avoid the race with schedule()->rcu_note_voluntary_context_switch(),
right?

> -		rcu_read_lock();
> -		do_each_thread(g, t) {
> -			if (t != current && ACCESS_ONCE(t->on_rq) &&
> -			    !is_idle_task(t)) {
> -				t->rcu_tasks_holdout = 1;

Because before this patch the code looks obviously racy, a task can do
sleep(FOREVER) and block rcu_tasks_kthread() if it reads ->on_rq == 1
after rcu_note_voluntary_context_switch() was already called.

However, I am not sure this race is actually closed even after this
change... why rcu_note_voluntary_context_switch() can not miss
->rcu_tasks_holdout != 0 ?

OK, it seems that you are going to send the next version anyway, so
please ignore.

Oleg.