[SCSI] Avoid dangling pointer in scsi_requeue_command()

When we call scsi_unprep_request() the command associated with the request
gets destroyed and therefore drops its reference on the device.  If this was
the only reference, the device may get released and we end up with a NULL
pointer deref when we call blk_requeue_request.
Reported-by: NMike Christie <michaelc@cs.wisc.edu>
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Reviewed-by: NMike Christie <michaelc@cs.wisc.edu>
Reviewed-by: NTejun Heo <tj@kernel.org>
Cc: <stable@kernel.org>
[jejb: enhance commend and add commit log for stable]
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_lib.c

@@ -479,15 +479,26 @@ void scsi_requeue_run_queue(struct work_struct *work)
  */
 static void scsi_requeue_command(struct request_queue *q, struct scsi_cmnd *cmd)
 {
+	struct scsi_device *sdev = cmd->device;
 	struct request *req = cmd->request;
 	unsigned long flags;
 
+	/*
+	 * We need to hold a reference on the device to avoid the queue being
+	 * killed after the unlock and before scsi_run_queue is invoked which
+	 * may happen because scsi_unprep_request() puts the command which
+	 * releases its reference on the device.
+	 */
+	get_device(&sdev->sdev_gendev);
+
 	spin_lock_irqsave(q->queue_lock, flags);
 	scsi_unprep_request(req);
 	blk_requeue_request(q, req);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 
 	scsi_run_queue(q);
+
+	put_device(&sdev->sdev_gendev);
 }
 
 void scsi_next_command(struct scsi_cmnd *cmd)