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

Paul E. McKenney
2009-06-24T14:05:09Z

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 <hawk-4UpuNZONu4c@public.gmane.org>
> > ---
> >
> > 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
> > + *
> > + * hawk-4UpuNZONu4c@public.gmane.org 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
>