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

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Message-ID: <20140730170442.GA29577@redhat.com>
Date:	Wed, 30 Jul 2014 19:04:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 8/9] rcu: Make RCU-tasks track exiting
	tasks

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.

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