linux-kernel - Re: [2.6.27-rc4] XFS i_lock vs i

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Message-ID: <20080826015549.GT5706@disturbed>
Date:	Tue, 26 Aug 2008 11:55:49 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lachlan McIlroy <lachlan@....com>,
	Daniel J Blueman <daniel.blueman@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, xfs@....sgi.com,
	hch@....de
Subject: Re: [2.6.27-rc4] XFS i_lock vs i_iolock...

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@...morbit.com
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