null_blk: restart request processing on completion handler

When irqmode=2 (IRQ completion handler is timer) and queue_mode=1
(Block interface to use is rq), the completion handler should restart
request handling for any pending requests on a queue because request
processing stops when the number of commands are queued more than
hw_queue_depth (null_rq_prep_fn returns BLKPREP_DEFER).

Without this change, the following command cannot finish.

	# modprobe null_blk irqmode=2 queue_mode=1 hw_queue_depth=1
	# fio --name=t --rw=read --size=1g --direct=1 \
	  --ioengine=libaio --iodepth=64 --filename=/dev/nullb0

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jens Axboe <axboe@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/null_blk.c

@@ -243,6 +243,17 @@ static enum hrtimer_restart null_cmd_timer_expired(struct hrtimer *timer)
 			cmd = container_of(entry, struct nullb_cmd, ll_list);
 			entry = entry->next;
 			end_cmd(cmd);
+
+			if (cmd->rq) {
+				struct request_queue *q = cmd->rq->q;
+
+				if (!q->mq_ops && blk_queue_stopped(q)) {
+					spin_lock(q->queue_lock);
+					if (blk_queue_stopped(q))
+						blk_start_queue(q);
+					spin_unlock(q->queue_lock);
+				}
+			}
 		} while (entry);
 	}
 
@@ -334,6 +345,7 @@ static int null_rq_prep_fn(struct request_queue *q, struct request *req)
 		req->special = cmd;
 		return BLKPREP_OK;
 	}
+	blk_stop_queue(q);
 
 	return BLKPREP_DEFER;
 }