linux-kernel - Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu

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Message-ID: <20140802230020.GD8101@linux.vnet.ibm.com>
Date:	Sat, 2 Aug 2014 16:00:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

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

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