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Date: Mon, 6 Aug 2018 18:45:04 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 6/6] xfs: avoid COW fork extent lookups in writeback if
        the fork didn't change
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To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@lst.de>, "Darrick J. Wong" <darrick.wong@oracle.com>, linux-xfs@vger.kernel.org

On Mon, Aug 06, 2018 at 12:37:38PM +1000, Dave Chinner wrote:
> IOWs, I can't see any reliable access serialisation being provided
> by holding the ILOCK while modifying the value. Perhaps this should
> use WRITE_ONCE/READ_ONCE to ensure the correct memory barriers are
> used to serialise tthe sequence count against itself rather than the
> wider inode extent modification process?

Yes, I think we need a barrier before the later actual modification
of the extent tree and thus should use WRITE_ONCE/READ_ONCE.

I've got a patch that is undergoing testing at the moment.