linux-kernel - Re: [-mm] Add an owner to the mm

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Message-ID: <47F5233F.1010108@linux.vnet.ibm.com>
Date:	Fri, 04 Apr 2008 00:04:39 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
CC:	Pavel Emelianov <xemul@...nvz.org>,
	Hugh Dickins <hugh@...itas.com>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, taka@...inux.co.jp,
	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [-mm] Add an owner to the mm_struct (v7)

Paul Menage wrote:
> On Thu, Apr 3, 2008 at 11:11 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>>  >
>>  > Hmm, is this new check for delay_group_leader() safe? Won't we have
>>  > called exit_cgroup() by this point, and hence be reassigned to the
>>  > root cgroup? And so mm->owner->cgroups won't point to the right place?
>>  >
>>
>>  cgroup_exit() comes in much later after exit_mm(). Moreover delay_group_leader()
>>  is a function that checks to see if
> 
> Sorry, I was unclear.
> 
> Yes, the call to cgroup_exit() comes much later than exit_mm() - but
> it probably does come before the other users of the mm have finished
> using the mm. So can't we end up with a situation like this?
> 
> A (group leader) exits; at this point, A->mm->owner == A
> A calls exit_mm(), sees delay_group_leader(), doesn't change A->mm->owner
> A calls cgroup_exit(), A->cgroups is set to init_css_set.
> B (another thread) does something with B->mm->owner->cgroups (e.g. VM
> accounting) and accesses the wrong group

Hi, Paul,

That is indeed quite bad. Do we have to retire the group_leader to init_css_set?
Can we not check for delay_group_leader() there?

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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