Re: [PATCH] xfs: prevent dropping ioend completions during buftarg wait

[Brian Foster <bfoster@redhat.com>]

On Tue, Sep 27, 2016 at 04:17:40PM +0200, Greg KH wrote:
> On Tue, Sep 27, 2016 at 10:03:57AM -0400, Brian Foster wrote:
> > commit 800b2694f890cc35a1bda63501fc71c94389d517 upstream.
> > 
> > xfs_wait_buftarg() waits for all pending I/O, drains the ioend
> > completion workqueue and walks the LRU until all buffers in the cache
> > have been released. This is traditionally an unmount operation` but the
> > mechanism is also reused during filesystem freeze.
> > 
> > xfs_wait_buftarg() invokes drain_workqueue() as part of the quiesce,
> > which is intended more for a shutdown sequence in that it indicates to
> > the queue that new operations are not expected once the drain has begun.
> > New work jobs after this point result in a WARN_ON_ONCE() and are
> > otherwise dropped.
> > 
> > With filesystem freeze, however, read operations are allowed and can
> > proceed during or after the workqueue drain. If such a read occurs
> > during the drain sequence, the workqueue infrastructure complains about
> > the queued ioend completion work item and drops it on the floor. As a
> > result, the buffer remains on the LRU and the freeze never completes.
> > 
> > Despite the fact that the overall buffer cache cleanup is not necessary
> > during freeze, fix up this operation such that it is safe to invoke
> > during non-unmount quiesce operations. Replace the drain_workqueue()
> > call with flush_workqueue(), which runs a similar serialization on
> > pending workqueue jobs without causing new jobs to be dropped. This is
> > safe for unmount as unmount independently locks out new operations by
> > the time xfs_wait_buftarg() is invoked.
> > 
> > cc: <stable@vger.kernel.org>
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > Signed-off-by: Dave Chinner <david@fromorbit.com>
> > ---
> > 
> > Note that this applies to linux-4.7.y.
> 
> It also applied to 4.4-stable, should I put it there as well?
> 

Yeah, it looks like we included "xfs: log mount failures don't wait for
buffers to be released" there so that probably makes sense. Thanks!

Brian

> thanks,
> 
> greg k-h