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Date: Tue, 26 Aug 2008 15:35:08 -0400
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [2.6.27-rc4] XFS i_lock vs i_iolock...
Message-ID: <20080826193508.GA17542@infradead.org>
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To: Christoph Hellwig <hch@lst.de>, Peter Zijlstra <peterz@infradead.org>, Lachlan McIlroy <lachlan@sgi.com>, Daniel J Blueman <daniel.blueman@gmail.com>, Linux Kernel <linux-kernel@vger.kernel.org>, xfs@oss.sgi.com

On Tue, Aug 26, 2008 at 12:45:47PM +1000, Dave Chinner wrote:
> XFS: prevent lockdep false positives when locking two inodes
> 
> If we call xfs_lock_two_inodes() to grab both the iolock and
> the ilock, then drop the ilocks on both inodes, then grab
> them again (as xfs_swap_extents() does) then lockdep will
> report a locking order problem. This is a false positive.
> 
> To avoid this, disallow xfs_lock_two_inodes() fom locking both
> inode locks at once - force calers to make two separate calls.
> This means that nested dropping and regaining of the ilocks
> will retain the same lockdep subclass and so lockdep will
> not see anything wrong with this code.

Looks good.  We probably don't need the #ifdef DEBUG as ASSERT is
debug-only anyway.