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Date:	Wed, 17 Nov 2010 09:24:01 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	stable@...nel.org, Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [BUGFIX] memcg: avoid deadlock between move charge and
 try_charge()

On Tue, 16 Nov 2010 12:41:17 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 16 Nov 2010 19:17:48 +0900
> Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> 
> > From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > 
> > __mem_cgroup_try_charge() can be called under down_write(&mmap_sem)(e.g.
> > mlock does it). This means it can cause deadlock if it races with move charge:
> > 
> > Ex.1)
> >                 move charge             |        try charge
> >   --------------------------------------+------------------------------
> >     mem_cgroup_can_attach()             |  down_write(&mmap_sem)
> >       mc.moving_task = current          |    ..
> >       mem_cgroup_precharge_mc()         |  __mem_cgroup_try_charge()
> >         mem_cgroup_count_precharge()    |    prepare_to_wait()
> >           down_read(&mmap_sem)          |    if (mc.moving_task)
> >           -> cannot aquire the lock     |    -> true
> >                                         |      schedule()
> > 
> > Ex.2)
> >                 move charge             |        try charge
> >   --------------------------------------+------------------------------
> >     mem_cgroup_can_attach()             |
> >       mc.moving_task = current          |
> >       mem_cgroup_precharge_mc()         |
> >         mem_cgroup_count_precharge()    |
> >           down_read(&mmap_sem)          |
> >           ..                            |
> >           up_read(&mmap_sem)            |
> >                                         |  down_write(&mmap_sem)
> >     mem_cgroup_move_task()              |    ..
> >       mem_cgroup_move_charge()          |  __mem_cgroup_try_charge()
> >         down_read(&mmap_sem)            |    prepare_to_wait()
> >         -> cannot aquire the lock       |    if (mc.moving_task)
> >                                         |    -> true
> >                                         |      schedule()
> > 
> > To avoid this deadlock, we do all the move charge works (both can_attach() and
> > attach()) under one mmap_sem section.
> > And after this patch, we set/clear mc.moving_task outside mc.lock, because we
> > use the lock only to check mc.from/to.
> > 
> > Signed-off-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> 
> I put this in the send-to-Linus-in-about-a-week queue.
> 
> > Cc: <stable@...nel.org>
> 
> The patch doesn't apply well to 2.6.36 so if we do want it backported
> then please prepare a tested backport for the -stable guys?
> 
O.K.
I'll test a backported patch for 2.6.36.y and send it after this is merged into mainline.

Thanks,
Daisuke Nishimura.
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