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

Paul E. McKenney
2014-08-03T22:05:43Z

On Sun, Aug 03, 2014 at 03:33:18PM +0200, Oleg Nesterov wrote:
> 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.

Whew! ;-)

> > 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. */
> }

OK, got it.

> > 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().

OK, looks easy enough. And yes, one good question is what, if anything,
we need to do with the idle threads.

> > 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().

As in create another global list that all tasks are added to when created
and then remved from when they exit?

Thanx, Paul