linux-kernel - Re: [PATCH] hugetlb/cgroup: Simplify pre

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Date:	Thu, 19 Jul 2012 16:56:18 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Li Zefan <lizefan@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	mhocko@...e.cz, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hugetlb/cgroup: Simplify pre_destroy callback

Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:

>>>>>
>>>>> We test RES_USAGE before taking hugetlb_lock.  What prevents some other
>>>>> thread from increasing RES_USAGE after that test?
>>>>>
>>>>> After walking the list we test RES_USAGE after dropping hugetlb_lock.
>>>>> What prevents another thread from incrementing RES_USAGE before that
>>>>> test, triggering the BUG?
>>>>
>>>> IIUC core cgroup will prevent a new task getting added to the cgroup
>>>> when we are in pre_destroy. Since we already check that the cgroup doesn't
>>>> have any task, the RES_USAGE cannot increase in pre_destroy.
>>>>
>>>
>>>
>>> You're wrong here. We release cgroup_lock before calling pre_destroy and retrieve
>>> the lock after that, so a task can be attached to the cgroup in this interval.
>>>
>>
>> But that means rmdir can be racy right ? What happens if the task got
>> added, allocated few pages and then moved out ? We still would have task
>> count 0 but few pages, which we missed to to move to parent cgroup.
>>
>
> That's a problem even if it's verrrry unlikely.
> I'd like to look into it and fix the race in cgroup layer.
> But I'm sorry I'm a bit busy in these days...
>

How about moving that mutex_unlock(&cgroup_mutex) to memcg callback ? That
can be a patch for 3.5 ? 

-aneesh
 

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