Re: [PATCH] cgroup: fix reverse unlock sequence in cgroup_get_sb

[Li Zefan <lizf@cn.fujitsu.com>]

>>> 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