From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx109.postini.com [74.125.245.109]) by kanga.kvack.org (Postfix) with SMTP id 42F356B004D for ; Fri, 20 Jan 2012 13:08:46 -0500 (EST) Received: by qadc11 with SMTP id c11so617881qad.14 for ; Fri, 20 Jan 2012 10:08:44 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20120119111727.6337bde4.kamezawa.hiroyu@jp.fujitsu.com> References: <20120113173001.ee5260ca.kamezawa.hiroyu@jp.fujitsu.com> <20120113173347.6231f510.kamezawa.hiroyu@jp.fujitsu.com> <20120117152635.GA22142@tiehlicka.suse.cz> <20120118090656.83268b3e.kamezawa.hiroyu@jp.fujitsu.com> <20120118123759.GB31112@tiehlicka.suse.cz> <20120119111727.6337bde4.kamezawa.hiroyu@jp.fujitsu.com> Date: Fri, 20 Jan 2012 10:08:44 -0800 Message-ID: Subject: Re: [RFC] [PATCH 2/7 v2] memcg: add memory barrier for checking account move. From: Ying Han Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: KAMEZAWA Hiroyuki Cc: Michal Hocko , "linux-mm@kvack.org" , "hugh.dickins@tiscali.co.uk" , "hannes@cmpxchg.org" , cgroups@vger.kernel.org, "bsingharora@gmail.com" On Wed, Jan 18, 2012 at 6:17 PM, KAMEZAWA Hiroyuki wrote: > On Wed, 18 Jan 2012 13:37:59 +0100 > Michal Hocko wrote: > >> On Wed 18-01-12 09:06:56, KAMEZAWA Hiroyuki wrote: >> > On Tue, 17 Jan 2012 16:26:35 +0100 >> > Michal Hocko wrote: >> > >> > > On Fri 13-01-12 17:33:47, KAMEZAWA Hiroyuki wrote: >> > > > I think this bugfix is needed before going ahead. thoughts? >> > > > =3D=3D >> > > > From 2cb491a41782b39aae9f6fe7255b9159ac6c1563 Mon Sep 17 00:00:00 = 2001 >> > > > From: KAMEZAWA Hiroyuki >> > > > Date: Fri, 13 Jan 2012 14:27:20 +0900 >> > > > Subject: [PATCH 2/7] memcg: add memory barrier for checking accoun= t move. >> > > > >> > > > At starting move_account(), source memcg's per-cpu variable >> > > > MEM_CGROUP_ON_MOVE is set. The page status update >> > > > routine check it under rcu_read_lock(). But there is no memory >> > > > barrier. This patch adds one. >> > > >> > > OK this would help to enforce that the CPU would see the current val= ue >> > > but what prevents us from the race with the value update without the >> > > lock? This is as racy as it was before AFAICS. >> > > >> > >> > Hm, do I misunderstand ? >> > =3D=3D >> > =A0 =A0update =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 reference >> > >> > =A0 =A0CPU A =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0CPU B >> > =A0 set value =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0rcu_read_lock() >> > =A0 smp_wmb() =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0smp_rmb() >> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0read_value >> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0rcu_read_unlock= () >> > =A0 synchronize_rcu(). >> > =3D=3D >> > I expect >> > If synchronize_rcu() is called before rcu_read_lock() =3D> move_lock_x= xx will be held. >> > If synchronize_rcu() is called after rcu_read_lock() =3D> update will = be delayed. >> >> Ahh, OK I can see it now. Readers are not that important because it is >> actually the updater who is delayed until all preexisting rcu read >> sections are finished. >> >> In that case. Why do we need both barriers? spin_unlock is a full >> barrier so maybe we just need smp_rmb before we read value to make sure >> that we do not get stalled value when we start rcu_read section after >> synchronize_rcu? >> > > I doubt .... If no barrier, this case happens > > =3D=3D > =A0 =A0 =A0 =A0update =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0reference > =A0 =A0 =A0 =A0CPU A =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 CPU B > =A0 =A0 =A0 =A0set value > =A0 =A0 =A0 =A0synchronize_rcu() =A0 =A0 =A0 rcu_read_lock() > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0read_value= <=3D find old value > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0rcu_read_u= nlock() > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0do no lock > =3D=3D Hi Kame, Can you help to clarify a bit more on the example above? Why read_value got the old value after synchronize_rcu(). Sorry for getting into this late. --Ying Sorry for getting into this late. > >> > Here, cpu B needs to read most recently updated value. >> >> If it reads the old value then it would think that we are not moving and >> so we would account to the old group and move it later on, right? >> > Right. without move_lock, we're not sure which old/new pc->mem_cgroup wil= l be. > This will cause mis accounting. > > > Thanks, > -Kame > > > > > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org