linux-kernel - Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu

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Date:	Fri, 1 Aug 2014 20:57:53 +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 v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On 07/31, Paul E. McKenney wrote:
>
> +		rcu_read_lock();
> +		for_each_process_thread(g, t) {
> +			if (t != current && ACCESS_ONCE(t->on_rq) &&
> +			    !is_idle_task(t)) {
> +				get_task_struct(t);
> +				t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
> +				ACCESS_ONCE(t->rcu_tasks_holdout) = 1;
> +				list_add(&t->rcu_tasks_holdout_list,
> +					 &rcu_tasks_holdouts);
> +			}
> +		}
> +		rcu_read_unlock();

So let me repeat. for_each_process_thread() above can not (in general) see
the exiting tasks which have already called exit_notify(), because such a
task can be removed from rcu lists at any time.

Now suppose that proc_exit_connector() is probed. Or another function which
can be called after exit_notify(), this doesn't matter.

An exiting task T jumps into trampoline and gets the preemption. It can be
already (auto)reaped and removed from rcu lists.

synchronize_rcu_tasks() can not see this task, so it can return before T gets
a chance to resume.

Oleg.

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