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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Mar 23, 2011 at 09:46:28AM -0400, Joe Korty wrote:
> On Wed, Mar 23, 2011 at 02:35:24AM -0400, Paul E. McKenney wrote:
> > On Mon, Mar 14, 2011 at 06:09:28PM -0400, Joe Korty wrote:
> > > 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.
> > 
> > Hello, Joe,
> > 
> > So the following grace period is not permitted to start until all of
> > the callbacks from the prior grace period have been invoked?
> 
> Yes, it is all synchronized.  Every cpu's current batch is
> closed off at the same time.  We tolerate some fuzz in the
> timing, but that is not the same as being unsynchronized.
> 
> This happens as an automatic side effect of doing
> everything (except for call_rcu) from a single CPU.

Got it -- the rcu_delimit_batches() doesn't return until all callbacks
have been invoked, and a new grace period cannot start until this function
returns.  I am not yet sure whether this is a good thing or a bad thing.
Given your 50-millisecond grace periods, it is less of a bad thing than
it would be for shorter grace periods, I suppose.  ;-)

							Thanx, Paul