Message-ID: <4A66630C.3030303@cn.fujitsu.com>
Date: Wed, 22 Jul 2009 08:53:32 +0800
From: Li Zefan <lizf@cn.fujitsu.com>
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To: balbir@linux.vnet.ibm.com
CC: Zefan Li <lizf.kernel@gmail.com>, Xiaotian Feng <dfeng@redhat.com>,
       menage@google.com, containers@lists.linux-foundation.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] cgroup: fix reverse unlock sequence in cgroup_get_sb
References: <1248171926-20232-1-git-send-email-dfeng@redhat.com> <20090721111019.GV24157@balbir.in.ibm.com> <8522a3d30907210438u6fce081fi835bf964d0c01e8a@mail.gmail.com> <20090721120106.GW24157@balbir.in.ibm.com>
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>>> 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@linux.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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