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

[Oleg Nesterov <oleg@redhat.com>]

On 08/01, Paul E. McKenney wrote:
>
> On Fri, Aug 01, 2014 at 04:11:44PM +0200, Oleg Nesterov wrote:
> > Not sure this makes any sense, but perhaps we can check for the new
> > callbacks and start the next gp. IOW, the main loop roughly does
> >
> > 	for (;;) {
> > 		list = rcu_tasks_cbs_head;
> > 		rcu_tasks_cbs_head = NULL;
> >
> > 		if (!list)
> > 			sleep();
> >
> > 		synchronize_sched();
> >
> > 		wait_for_rcu_tasks_holdout();
> >
> > 		synchronize_sched();
> >
> > 		process_callbacks(list);
> > 	}
> >
> > we can "join" 2 synchronize_sched's and do
> >
> > 	ready_list = NULL;
> > 	for (;;) {
> > 		list = rcu_tasks_cbs_head;
> > 		rcu_tasks_cbs_head = NULL;
> >
> > 		if (!list && !ready_list)
> > 			sleep();
> >
> > 		synchronize_sched();
> >
> > 		if (ready_list) {
> > 			process_callbacks(ready_list);
> > 			ready_list = NULL;
> > 		}
> >
> > 		if (!list)
> > 			continue;
> >
> > 		wait_for_rcu_tasks_holdout();
> > 		ready_list = list;
> > 	}
>
> The lack of barriers for the updates I am checking mean that I really
> do need a synchronize_sched() on either side of the grace-period wait.

Yes,

> The grace period needs to guarantee that anything that happened on any
> CPU before the start of the grace period happens before anything that
> happens on any CPU after the end of the grace period.  If I leave off
> either synchronize_sched(), we lose this guarantee.

But the 2nd variant still has synchronize_sched() on both sides?

Oleg.