Re: [PATCH 4/4] xfs: implement freezing by emptying the AIL

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

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.