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

[Christoph Hellwig <hch@infradead.org>]

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.
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