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

Jens Axboe
2007-06-17T07:30:15Z

On Sat, Jun 16 2007, Christoph Hellwig wrote:
> 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.

The actual commit text is very good though, but I agree - I don't think
the url comment is worth anything. I did consider just killing it.
However, the comment does describe the problem, so I think it's still
ok.

--
Jens Axboe