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

[Jeff Garzik <jeff@garzik.org>]

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 or 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>

Acked-by: Jeff Garzik <jeff@garzik.org>

Thanks Tejun, you kick ass as usual.

	Jeff