linux-kernel - Re: [PATCH 8/9] jrcu: fix broken rcu

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Message-ID: <20110314220928.GA1348@tsunami.ccur.com>
Date:	Mon, 14 Mar 2011 18:09:28 -0400
From:	Joe Korty <joe.korty@...r.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"fweisbec@...il.com" <fweisbec@...il.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"laijs@...fujitsu.com" <laijs@...fujitsu.com>,
	"mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"loic.minier@...aro.org" <loic.minier@...aro.org>,
	"dhaval.giani@...il.com" <dhaval.giani@...il.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"josh@...htriplett.org" <josh@...htriplett.org>,
	"houston.jim@...cast.net" <houston.jim@...cast.net>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/9] jrcu: fix broken rcu_barrier

On Mon, Mar 14, 2011 at 04:47:18AM -0400, Paul E. McKenney wrote:
> On Sun, Mar 13, 2011 at 11:50:44PM -0400, Joe Korty wrote:
> > +void rcu_barrier(void)
> > +{
> > +	synchronize_sched();
> > +	synchronize_sched();
> > +}
> 
> This is still broken.  You need to wait until all pre-existing RCU
> callbacks on all CPUs have been invoked.  Although this does guarantee
> that all callbacks on the current CPU have been invoked, if one of the
> other CPUs is busy, it might not yet have processed the callbacks from
> earlier grace periods.  The usual way to take care of this is to post
> a callback on each CPU, then wait for them all to be invoked.

Hi Paul,
JRCU (eventually) chains together all callbacks from all
cpus into a single queue and then it expires that single
queue.  So the double synchronize_sched() should work as
a rcu_barrier implementation.

Thanks,
Joe
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