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Re: [2.6.27-rc4] XFS i_lock vs i_iolock...

To: Lachlan McIlroy <lachlan@xxxxxxx>
Subject: Re: [2.6.27-rc4] XFS i_lock vs i_iolock...
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 25 Aug 2008 13:55:42 +1000
Cc: Daniel J Blueman <daniel.blueman@xxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, hch@xxxxxx
In-reply-to: <48B21507.9050708@xxxxxxx>
Mail-followup-to: Lachlan McIlroy <lachlan@xxxxxxx>, Daniel J Blueman <daniel.blueman@xxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, hch@xxxxxx
References: <6278d2220808221412x28f4ac5dl508884c8030b364a@xxxxxxxxxxxxxx> <20080825010213.GO5706@disturbed> <48B21507.9050708@xxxxxxx>
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On Mon, Aug 25, 2008 at 12:12:23PM +1000, Lachlan McIlroy wrote:
> Dave Chinner wrote:
>> On Fri, Aug 22, 2008 at 10:12:59PM +0100, Daniel J Blueman wrote:
>>> On 2.6.27-rc4 with various debug options enabled, lockdep claims lock
>>> ordering issues with XFS [1] - easiest reproducer is just running
>>> xfs_fsr. Mount options I was using were
>>> 'nobarrier,noatime,nodiratime'.
>>> Thanks,
>>>   Daniel
>>> --- [1]
>>> =======================================================
>>> [ INFO: possible circular locking dependency detected ]
>>> 2.6.27-rc4-224c #1
>>> -------------------------------------------------------
>>> xfs_fsr/5763 is trying to acquire lock:
>>>  (&(&ip->i_lock)->mr_lock/2){--..}, at: [<ffffffff803ad8fc>] 
>>> xfs_ilock+0x8c/0xb0
>>> but task is already holding lock:
>>>  (&(&ip->i_iolock)->mr_lock/3){--..}, at: [<ffffffff803ad915>]
>>> xfs_ilock+0xa5/0xb0
>> False positive. We do:
>>      xfs_lock_two_inodes(ip, tip, XFS_IOLOCK_EXCL | XFS_ILOCK_EXCL);
> Why not just change the above line to two lines:
>       xfs_lock_two_inodes(ip, tip, XFS_IOLOCK_EXCL);
>       xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);

Yeah, that'd work, but it implllies that we no longer allow
xfs_lock_two_inodes() to take both inode locks at once. It
would need a comment blaming^Wexplaining why lockdep requires
us to do this, and then debug code in xfs_lock_two_inodes() to
catch this when someone makes this mistake again in the future.


Dave Chinner

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