Re: [RFC] [PATCH 2/7 v2] memcg: add memory barrier for checking account move.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 18 Jan 2012 13:37:59 +0100
Michal Hocko <mhocko@suse.cz> wrote:

> On Wed 18-01-12 09:06:56, KAMEZAWA Hiroyuki wrote:
> > On Tue, 17 Jan 2012 16:26:35 +0100
> > Michal Hocko <mhocko@suse.cz> 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@jp.fujitsu.com>
> > > > 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





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