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

Oleg Nesterov
2014-08-03T13:36:03Z

On 08/02, Paul E. McKenney wrote:
>
> On Sat, Aug 02, 2014 at 04:56:16PM +0200, Oleg Nesterov wrote:
> > 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)) {
> >
> > I didn't notice this check before, but it is not needed. for_each_process()
> > can see the idle threads, there are not on process/thread lists.
^^^^^^^

argh, I mean't "can't see" of course...

> Good to know. Any other important tasks I am missing?

Nothing else.

> I am guessing that I need to do something like this:
>
> for_each_process(g) {
> /* Do build step. */
> for_each_thread(g, t) {
> if (g == t)
> continue;
> /* Do build step. */
> }
> }

Sorry, I don't understand... This is equivalent to

for_each_process_thread(g, t) {
/* Do build step. */
}

> Your point about exiting tasks I get, and I believe I can solve it.
> I am hoping that the above sort of construction takes care of the
> idle threads.

It is simple to find the idle threads, just

for_each_cpu(cpu) {
do_something(cpu_rq(cpu)->idle);
}

but it is not clear to me what else you need to do with the idle threads.
Probably not too much, they mostly run under preempt_disable().

> I might also need to do something to handle changes in
> process/thread hierarchy -- but hopefully without having to read-acquire
> the task-list lock.

It seems that you need another global list, a task should be visible on that
list until exit_rcu().

Oleg.