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

To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Subject: Re: [2.6.27-rc4] XFS i_lock vs i_iolock...
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 26 Aug 2008 11:55:49 +1000
Cc: Lachlan McIlroy <lachlan@xxxxxxx>, Daniel J Blueman <daniel.blueman@xxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, hch@xxxxxx
In-reply-to: <1219647573.20732.28.camel@twins>
Mail-followup-to: Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Lachlan McIlroy <lachlan@xxxxxxx>, Daniel J Blueman <daniel.blueman@xxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, hch@xxxxxx
References: <6278d2220808221412x28f4ac5dl508884c8030b364a@mail.gmail.com> <20080825010213.GO5706@disturbed> <48B21507.9050708@sgi.com> <20080825035542.GR5706@disturbed> <1219647573.20732.28.camel@twins>
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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@xxxxxxxxxxxxx


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