Re: [PATCH 8/9] jrcu: fix broken rcu_barrier

[Joe Korty <joe.korty@ccur.com>]

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