Re: [PATCH] block: always requeue !fs requests at the front

Christoph Hellwig
2007-06-16T20:19:55Z

On Fri, Jun 15, 2007 at 01:05:44PM +0200, Jens Axboe wrote:
> On Fri, Jun 15 2007, Tejun Heo wrote:
> > SCSI marks internal commands with REQ_PREEMPT and push it at the front
> > of the request queue using blk_execute_rq(). When entering suspended
> > or frozen state, SCSI devices are quiesced using
> > scsi_device_quiesce(). In quiesced state, only REQ_PREEMPT requests
> > are processed. This is how SCSI blocks other requests out while
> > suspending and resuming. As all internal commands are pushed at the
> > front of the queue, this usually works.
> >
> > Unfortunately, this interacts badly with ordered requeueing. To
> > preserve request order on requeueing (due to busy device, active EH or
> > other failures), requests are sorted according to ordered sequence on
> > requeue if IO barrier is in progress.
> >
> > The following sequence deadlocks.
> >
> > 1. IO barrier sequence issues.
> >
> > 2. Suspend requested. Queue is quiesced with part of all of IO
> > barrier sequence at the front.
> >
> > 3. During suspending or resuming, SCSI issues internal command which
> > gets deferred and requeued for some reason. As the command is
> > issued after the IO barrier in #1, ordered requeueing code puts the
> > request after IO barrier sequence.
> >
> > 4. The device is ready to process requests again but still is in
> > quiesced state and the first request of the queue isn't
> > REQ_PREEMPT, so command processing is deadlocked -
> > suspending/resuming waits for the issued request to complete while
> > the request can't be processed till device is put back into
> > running state by resuming.
> >
> > This can be fixed by always putting !fs requests at the front when
> > requeueing.
> >
> > The following thread reports this deadlock.
> >
> > http://thread.gmane.org/gmane.linux.kernel/537473
> >
> > Signed-off-by: Tejun Heo <htejun@gmail.com>
> > Cc: Jenn Axboe <jens.axboe@oracle.com>
> > Cc: David Greaves <david@dgreaves.com>
> > ---
> > Okay, it took a lot of hours of debugging but boiled down to two liner
> > fix. I feel so empty. :-) RAID6 triggers this reliably because it
> > uses BIO_BARRIER heavily to update its superblock. The recent ATA
> > suspend/resume rewrite is hit by this because it uses SCSI internal
> > commands to spin down and up the drives for suspending and resuming.
> >
> > David, please test this. Jens, does it look okay?
>
> Yep looks good, except for the bad multi-line comment style, but that's
> minor stuff ;-)
>
> Acked-by: Jens Axboe <jens.axboe@oracle.com>

I'd much much prefer having a description of the problem in the actual
comment then a hyperlink. There's just too much chance of the latter
breaking over time, and it's impossible to update it when things change
that should be reflected in the comment.