From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751722Ab1FHSnL (ORCPT ); Wed, 8 Jun 2011 14:43:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:3353 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750912Ab1FHSnJ (ORCPT ); Wed, 8 Jun 2011 14:43:09 -0400 Date: Wed, 8 Jun 2011 14:42:54 -0400 From: Vivek Goyal To: Paul Bolle Cc: Jens Axboe , "Paul E. McKenney" , linux-kernel@vger.kernel.org Subject: Re: [PATCH 5/5] CFQ: use proper locking for cache of last hit cic Message-ID: <20110608184254.GD1150@redhat.com> References: <1307291201.8545.46.camel@t41.thuisdomein> <4DEC441C.9030009@kernel.dk> <1307557130.2783.5.camel@t41.thuisdomein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1307557130.2783.5.camel@t41.thuisdomein> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 08, 2011 at 08:18:44PM +0200, Paul Bolle wrote: > On Mon, 2011-06-06 at 05:06 +0200, Jens Axboe wrote: > > On 2011-06-05 18:26, Paul Bolle wrote: > > > @@ -2704,8 +2706,13 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd, > > > smp_wmb(); > > > cic->key = cfqd_dead_key(cfqd); > > > > > > - if (ioc->last_cic == cic) > > > + spin_lock_irqsave(&ioc->lock, flags); > > > + rcu_read_lock(); > > > + last_cic = rcu_dereference(ioc->last_cic); > > > + rcu_read_unlock(); > > > + if (last_cic == cic) > > > rcu_assign_pointer(ioc->last_cic, NULL); > > > + spin_unlock_irqrestore(&ioc->lock, flags); > > > > We don't need the ioc->lock for checking the cache, it would in fact > > defeat the purpose of using RCU. > > Just to show that I'm RCU-challenged, is that because: > 1) my use of locking on ioc->lock defends for a race that is not > actually possible; or > 2) the worst thing that could happen is that some new and correct value > of ioc->last_cic will be replaced with NULL, which is simply not a big > deal? I don't understand this point. All ioc->ioc_data updates are under ioc->lock except the one __cfq_exit_single_io_context() and that's what jens patch fixed. So clearly there was atleast one race where we were doing a value update without taking appropriate lock. Why do you think that some new and correct value will be replaced by NULL? Thanks Vivek