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

From: Jens Axboe
Date: Fri Jun 15 2007 - 07:05:43 EST

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.
> Signed-off-by: Tejun Heo <htejun@xxxxxxxxx>
> Cc: Jenn Axboe <jens.axboe@xxxxxxxxxx>
> Cc: David Greaves <david@xxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>

Jens Axboe

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