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Date: Sat, 24 Mar 2012 13:08:48 -0400
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 4/4] xfs: implement freezing by emptying the AIL
Message-ID: <20120324170848.GA32591@infradead.org>
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To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>, xfs@oss.sgi.com

On Sat, Mar 24, 2012 at 01:06:18PM -0400, Christoph Hellwig wrote:
> On Thu, Mar 22, 2012 at 10:57:38AM +1100, Dave Chinner wrote:
> > > +	 * threshold.
> > > +	 */
> > > +	if (atomic_read(&ailp->xa_wait_empty))
> > > +		target = xfs_ail_max(ailp)->li_lsn;
> > 
> > I don't think this is safe - we may have finished pushing the AIL to
> > empty, but the waiter that decrements xa_wait_empty may not have run
> > yet and we can race with that. In that case, xfs_ail_max() will
> > return NULL as the AIL is empty.
> 
> True - I'll add a check for that.

Actually - we do the xfs_trans_ail_cursor_first check just above - if
the AIL is empty we'll already exit after that and never reach this
code, so we should be fine.

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