Messages in this thread | | | Date | Tue, 26 Aug 2008 11:55:49 +1000 | From | Dave Chinner <> | Subject | Re: [2.6.27-rc4] XFS i_lock vs i_iolock... |
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On Mon, Aug 25, 2008 at 08:59:33AM +0200, Peter Zijlstra wrote: > On Mon, 2008-08-25 at 13:55 +1000, Dave Chinner wrote: > > 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: > > >>> ======================================================= > > >>> [ 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. > > How can you take two locks in one go? It seems to me you always need to > take them one after another, and as soon as you do that, you have > ordering constraints.
It doesn't take them both inode locks in one go - it does them separately in a given order via xfs_ilock(). Basically there are two layers of constraints here - xfs_ilock() handles the order withing a given inode, xfs_lock_two_inodes() handles order and deadlock prevention between inodes.
What lockdep is complaining about is a difference in the lock order between different locks in different inodes - a situation that does not result in a deadlock...
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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