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

[Ying Han <yinghan@google.com>]

On Wed, Jan 18, 2012 at 6:17 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 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
> ==

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 will be.
> This will cause mis accounting.
>
>
> Thanks,
> -Kame
>
>
>
>
>

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