From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx146.postini.com [74.125.245.146]) by kanga.kvack.org (Postfix) with SMTP id 983DD6B004D for ; Mon, 23 Jan 2012 04:04:41 -0500 (EST) Date: Mon, 23 Jan 2012 10:04:36 +0100 From: Michal Hocko Subject: Re: [RFC] [PATCH 2/7 v2] memcg: add memory barrier for checking account move. Message-ID: <20120123090436.GA12375@tiehlicka.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Ying Han Cc: KAMEZAWA Hiroyuki , "linux-mm@kvack.org" , "hugh.dickins@tiscali.co.uk" , "hannes@cmpxchg.org" , cgroups@vger.kernel.org, "bsingharora@gmail.com" On Fri 20-01-12 10:08:44, Ying Han wrote: > 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? > >> > > > == > >> > > > 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 account 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 value > >> > > 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 ? > >> > == > >> > update reference > >> > > >> > CPU A CPU B > >> > set value rcu_read_lock() > >> > smp_wmb() smp_rmb() > >> > read_value > >> > rcu_read_unlock() > >> > synchronize_rcu(). > >> > == > >> > I expect > >> > If synchronize_rcu() is called before rcu_read_lock() => move_lock_xxx will be held. > >> > If synchronize_rcu() is called after rcu_read_lock() => 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 > > > > == > > update reference > > CPU A CPU B > > set value > > synchronize_rcu() rcu_read_lock() > > read_value <= find old value > > rcu_read_unlock() > > do no lock > > == > > Hi Kame, > > Can you help to clarify a bit more on the example above? Why > read_value got the old value after synchronize_rcu(). AFAIU it is because rcu_read_unlock doesn't force any memory barrier and we synchronize only the updater (with synchronize_rcu), so nothing guarantees that the value set on CPUA is visible to CPUB. -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- 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