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

To: Daniel J Blueman <daniel.blueman@xxxxxxxxx>
Subject: Re: [2.6.27-rc4] XFS i_lock vs i_iolock...
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 25 Aug 2008 11:02:13 +1000
Cc: Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, hch@xxxxxx
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Mail-followup-to: Daniel J Blueman <daniel.blueman@xxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, hch@xxxxxx
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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);
        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);
        xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);

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...


Dave Chinner

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