On Wed, 18 Jan 2012 13:37:59 +0100 Michal Hocko <mhocko@xxxxxxx> wrote: > On Wed 18-01-12 09:06:56, KAMEZAWA Hiroyuki wrote: > > On Tue, 17 Jan 2012 16:26:35 +0100 > > Michal Hocko <mhocko@xxxxxxx> 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 <kamezawa.hiroyu@xxxxxxxxxxxxxx> > > > > 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 == > > 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 will be. This will cause mis accounting. Thanks, -Kame -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>