[Ocfs2-devel] [patch 04/11] ocfs2: fix a tiny race when running dirop_fileop_racer

[Xue jiufei]

Hi, Mark
On 2014/2/6 7:31, Mark Fasheh wrote:
> On Fri, Jan 24, 2014 at 12:47:03PM -0800, akpm at linux-foundation.org wrote:
>> From: Yiwen Jiang <jiangyiwen at huawei.com>
>> Subject: ocfs2: fix a tiny race when running dirop_fileop_racer
>>
>> When running dirop_fileop_racer we found a dead lock case.
>>
>> 2 nodes, say Node A and Node B, mount the same ocfs2 volume.  Create
>> /race/16/1 in the filesystem, and let the inode number of dir 16 is less
>> than the inode number of dir race.
>>
>> Node A                            Node B
>> mv /race/16/1 /race/
>>                                   right after Node A has got the
>>                                   EX mode of /race/16/, and tries to
>>                                   get EX mode of /race
>>                                   ls /race/16/
>>
>> In this case, Node A has got the EX mode of /race/16/, and wants to get EX
>> mode of /race/.  Node B has got the PR mode of /race/, and wants to get
>> the PR mode of /race/16/.  Since EX and PR are mutually exclusive, dead
>> lock happens.
> 
> I am confused as to how this race happens.
> 
> Something like "ls /race/16' shouldn't hold locks on 'race' and '16' at the
> same time. It should look more like:
> 
> <userspace does readdir /race/16>
> PR race
> <kernel looks up '16' in 'race'>
> Unlock PR race
> PR 16
> <get dirents from '16'>
> Unlock PR 16
> <return dirents to userspace>
> 
> Can you please explain where I may be going wrong? Also an strace of the
> locked up 'ls' as well as the output of sysrq-t when it's deadlocked would
> help show what's going on.
> 	--Mark
> 
when doing 'ls /race/16', it calls vfs_fstatat->..->d_alloc()->ocfs2_lookup()
after readdir(). ocfs2_lookup() first get PR lock of race, and then get PR
lock of 16 in ocfs2_iget() without unlocking PR race.
	-- joyce.xue
> --
> Mark Fasheh
> 
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