[SCSI] make blk layer set REQ_SOFTBARRIER on defer and requeue

This is the reworked version of the patch.  It sets REQ_SOFTBARRIER
in two places - in elv_next_request() on BLKPREP_DEFER and in
blk_requeue_request().
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

drivers/block/elevator.c

@@ -290,6 +290,13 @@ void elv_requeue_request(request_queue_t *q, struct request *rq)
 		rq = rq->end_io_data;
 	}
 
+	/*
+	 * the request is prepped and may have some resources allocated.
+	 * allowing unprepped requests to pass this one may cause resource
+	 * deadlock.  turn on softbarrier.
+	 */
+	rq->flags |= REQ_SOFTBARRIER;
+
 	/*
 	 * if iosched has an explicit requeue hook, then use that. otherwise
 	 * just put the request at the front of the queue
@@ -386,6 +393,12 @@ struct request *elv_next_request(request_queue_t *q)
 		if (ret == BLKPREP_OK) {
 			break;
 		} else if (ret == BLKPREP_DEFER) {
+			/*
+			 * the request may have been (partially) prepped.
+			 * we need to keep this request in the front to
+			 * avoid resource deadlock.  turn on softbarrier.
+			 */
+			rq->flags |= REQ_SOFTBARRIER;
 			rq = NULL;
 			break;
 		} else if (ret == BLKPREP_KILL) {