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

[Jeff Garzik]

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@xxxxxxxxx>
> Cc: Jenn Axboe <jens.axboe@xxxxxxxxxx>
> Cc: David Greaves <david@xxxxxxxxxxxx>

Acked-by: Jeff Garzik <jeff@xxxxxxxxxx>

Thanks Tejun, you kick ass as usual.

	Jeff



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