linux-kernel - Re: [PATCH] cgroup: fix reverse unlock sequence in cgroup_get

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Date:	Wed, 22 Jul 2009 08:53:32 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
CC:	Zefan Li <lizf.kernel@...il.com>, Xiaotian Feng <dfeng@...hat.com>,
	menage@...gle.com, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: fix reverse unlock sequence in cgroup_get_sb

>>> Seems reasonable to me. You might also want to mention that elsewhere
>>> the sequence is unlock cgroup_mutex followed by inode->i_mutex.
>>>
>>> Acked-by: Balbir Singh balbir@...ux.vnet.ibm.com
>>
>> No, the unlock order is irrelevant. It's the lock order that matters. So
>> this patch
>> fixes nothing.
>>
>> Xiaotian, you didn't run into deadlock, did you?
>>
> 
> 
> Li, Consider the following
> 
> 
> lock(A)
> lock(B)
> unlock(A) 
> unlock(B)
> 
> Tomorrow if a unsuspecting programmer does this
> 
> lock(A)
> lock(B)
> unlock(A) 
> 
> code block 
> 
> unlock(B)
> 
> 
> What protects code block? lock B? Is that the intention?
> 

I won't worry about that. If unlock order is a concern,
we should have taught lockdep to detect it.

Here's a reply from Linus on this issue:

	http://lkml.org/lkml/2008/10/8/150
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