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

Oleg Nesterov
2014-08-01T19:00:51Z

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.