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

[Jens Axboe <jens.axboe@oracle.com>]

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