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

[Dave Chinner <david@fromorbit.com>]

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);
		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.
-- 
Dave Chinner
david@fromorbit.com