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Date:	Wed, 17 Nov 2010 09:11:35 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [BUGFIX] memcg: avoid deadlock between move charge and
 try_charge()

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>
> Cc: <stable@...nel.org>

Thanks,
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

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