From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755008AbZDULja (ORCPT ); Tue, 21 Apr 2009 07:39:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753192AbZDULjT (ORCPT ); Tue, 21 Apr 2009 07:39:19 -0400 Received: from fms-01.valinux.co.jp ([210.128.90.1]:52448 "EHLO mail.valinux.co.jp" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753367AbZDULjS (ORCPT ); Tue, 21 Apr 2009 07:39:18 -0400 Date: Tue, 21 Apr 2009 20:39:17 +0900 (JST) Message-Id: <20090421.203917.189725396.ryov@valinux.co.jp> To: righi.andrea@gmail.com Cc: balbir@linux.vnet.ibm.com, randy.dunlap@oracle.com, axboe@kernel.dk, dradford@bluehost.com, akpm@linux-foundation.org, ngupta@google.com, fernando@oss.ntt.co.jp, linux-kernel@vger.kernel.org, chlunde@ping.uio.no, dave@linux.vnet.ibm.com, roberto@unbit.it, agk@sourceware.org, matt@bluehost.com, containers@lists.linux-foundation.org, menage@google.com, subrata@linux.vnet.ibm.com, eric.rannaud@gmail.com Subject: Re: [PATCH 3/9] bio-cgroup controller From: Ryo Tsuruta In-Reply-To: <20090420145658.GB13307@linux> References: <20090417102214.GC3896@balbir.in.ibm.com> <20090420.203540.104031006.ryov@valinux.co.jp> <20090420145658.GB13307@linux> X-Mailer: Mew version 5.2.52 on Emacs 22.1 / Mule 5.0 (SAKAKI) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Andrea, > On Mon, Apr 20, 2009 at 08:35:40PM +0900, Ryo Tsuruta wrote: > > > > +/* > > > > + * Assign "page" the same owner as "opage." > > > > + */ > > > > +void bio_cgroup_copy_owner(struct page *npage, struct page *opage) > > > > +{ > > > > + struct page_cgroup *npc, *opc; > > > > + > > > > + if (bio_cgroup_disabled()) > > > > + return; > > > > + npc = lookup_page_cgroup(npage); > > > > + if (unlikely(!npc)) > > > > + return; > > > > + opc = lookup_page_cgroup(opage); > > > > + if (unlikely(!opc)) > > > > + return; > > > > + > > > > + /* > > > > + * Do this without any locks. The reason is the same as > > > > + * bio_cgroup_reset_owner(). > > > > + */ > > > > + npc->bio_cgroup_id = opc->bio_cgroup_id; > > > > > > What protects npc and opc? > > > > As the same reason mentioned above, bio_cgroup_id can be updated > > without any locks, and npc and opc always point to page_cgroups. > > An integer variable can be set a new value at once on a system which > > can use RCU lock. > > mmmh... I'm not sure about this. Actually you read opc->bio_cgroup_id > first and then write to npc->bio_cgroup_id, so it is not atomic at all. > So, you can read or set a wrong ID, but at least it should be always > consistent (the single read or write itself is atomic). Even if opc->bio_cgroup_id is changed before copying to npc->bio_cgroup_id, npc->bio_cgroup_id is correctly updated at some time. The implementation is not completely accurate, but is faster and lighter. Thanks, Ryo Tsuruta