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.
Here, cpu B needs to read most recently updated value.
Thanks,
-Kame
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