Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

[Oleg Nesterov]

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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