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

To: Daniel J Blueman <daniel.blueman@xxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, hch@xxxxxx
Subject: Re: [2.6.27-rc4] XFS i_lock vs i_iolock...
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Mon, 25 Aug 2008 12:12:23 +1000
In-reply-to: <20080825010213.GO5706@disturbed>
References: <6278d2220808221412x28f4ac5dl508884c8030b364a@xxxxxxxxxxxxxx> <20080825010213.GO5706@disturbed>
Reply-to: lachlan@xxxxxxx
Sender: xfs-bounce@xxxxxxxxxxx
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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);


        .....
        xfs_iunlock(ip, XFS_ILOCK_EXCL);
        xfs_iunlock(tip, XFS_ILOCK_EXCL);
        .....
        xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);

Which is a perfectly valid thing to do.

The problem is that lockdep is complaining about the second call
to xfs_lock_two_inodes(), which uses the subclasses 2 and 3.
effectively it is seeing:

        xfs_lock_two_inodes(ip, tip, XFS_IOLOCK_EXCL | XFS_ILOCK_EXCL);
                iolock/2
                ilock/2
                iolock/3
                ilock/3
        .....
        xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);
                ilock/2
                ilock/3


But because the original lock order was ilock/2->iolock/3, the
second call to xfs_lock_two_inodes is seeing iolock/3->ilock/2
which it then complains about....

Christoph - I think we're going to need to pass a lockdep 'order'
flag into xfs_lock_two_inodes() to avoid this so the second call
can use different classes to the first call. Or perhaps a '_nested'
variant of the call...

Cheers,

Dave.


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