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

Paul E. McKenney
2014-08-02T23:00:28Z

On Fri, Aug 01, 2014 at 08:40:59PM +0200, Oleg Nesterov wrote:
> 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?

Your second variant above? Unless it is in wait_for_rcu_tasks_holdouts(),
I am not seeing it.

Thanx, Paul