Subject: Re: [PATCH 09/10] cfq-iosched: Uses its own open-coded rcu_barrier.

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 24 Jun 2009 07:05:05 -0700

On Wed, Jun 24, 2009 at 08:42:37AM +0200, Jens Axboe wrote:
> On Tue, Jun 23 2009, Jesper Dangaard Brouer wrote:
> > This module cfq-iosched, has discovered the value of waiting for
> > call_rcu() completion, but its has its own open-coded implementation
> > of rcu_barrier(), which I don't think is 'strong' enough.
> > 
> > This patch only leaves a comment for the maintainers to consider.
> 
> We need a stronger primitive and rcu_barrier(), since we also need to
> wait for the rcu calls to even be scheduled. So I don't think the below
> can be improved, it's already fine.

It is indeed important to first prevent new call_rcu() instances from
being invoked, and only then invoke rcu_barrier().

							Thanx, Paul

> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> > 
> >  block/cfq-iosched.c |    6 ++++++
> >  1 files changed, 6 insertions(+), 0 deletions(-)
> > 
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index 833ec18..c15555b 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -2657,6 +2657,12 @@ static void __exit cfq_exit(void)
> >  	/*
> >  	 * this also protects us from entering cfq_slab_kill() with
> >  	 * pending RCU callbacks
> > +	 *
> > +	 * [email-masked] 2009-06-18: Maintainer please consider using
> > +	 *  rcu_barrier() instead of this open-coded wait for
> > +	 *  completion implementation.  I think it provides a better
> > +	 *  guarantee that all CPUs are finished, although
> > +	 *  elv_ioc_count_read() do consider all CPUs.
> >  	 */
> >  	if (elv_ioc_count_read(ioc_count))
> >  		wait_for_completion(&all_gone);
> > 
> 
> -- 
> Jens Axboe
> 
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