From: "Paul E. McKenney" Subject: Re: [PATCH 09/10] cfq-iosched: Uses its own open-coded rcu_barrier. Date: Wed, 24 Jun 2009 07:05:05 -0700 Message-ID: <20090624140505.GA6779@linux.vnet.ibm.com> References: <20090623150330.22490.87327.stgit@localhost> <20090623150439.22490.14657.stgit@localhost> <20090624064236.GE31415@kernel.dk> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jesper Dangaard Brouer , "David S. Miller" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, dougthompson@xmission.com, bluesmoke-devel@lists.sourceforge.net, Patrick McHardy , christine.caulfield@googlemail.com, Trond.Myklebust@netapp.com, linux-wireless@vger.kernel.org, johannes@sipsolutions.net, yoshfuji@linux-ipv6.org, shemminger@linux-foundation.org, linux-nfs@vger.kernel.org, bfields@fieldses.org, neilb@suse.de, linux-ext4@vger.kernel.org, tytso@mit.edu, adilger-xsfywfwIY+M@public.gmane.org, netfilter-devel@vger.kernel.org To: Jens Axboe Return-path: Received: from e7.ny.us.ibm.com ([32.97.182.137]:47826 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751240AbZFXOFJ (ORCPT ); Wed, 24 Jun 2009 10:05:09 -0400 In-Reply-To: <20090624064236.GE31415@kernel.dk> Sender: linux-nfs-owner@vger.kernel.org List-ID: 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 > > --- > > > > 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 >