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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Aug 04, 2014 at 08:37:37AM +0800, Lai Jiangshan wrote:
> On 08/04/2014 06:05 AM, Paul E. McKenney wrote:
> > 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?
> 
> An alternative solution:
> srcu_read_lock() before exit_notify(), srcu_read_unlock() after the last preempt_disable()
> in the do_exit, and synchronize_srcu() in rcu_tasks_kthread().

That is a good way to synchronize with the exiting tasks, and I will
probably that that approach.

I -thought- that Oleg was concerned about safely building the list to
start with, though.

							Thanx, Paul