nvme-loop: fix kernel oops in case of unhandled command

When nvmet_req_init() fails, __nvmet_req_complete() is called
to handle the target request via .queue_response(), so
nvme_loop_queue_response() shouldn't be called again for
handling the failure.

This patch fixes this case by the following way:

- move blk_mq_start_request() before nvmet_req_init(), so
nvme_loop_queue_response() may work well to complete this
host request

- don't call nvme_cleanup_cmd() which is done in nvme_loop_complete_rq()

- don't call nvme_loop_queue_response() which is done via
.queue_response()

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[trimmed changelog]
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/target/loop.c

@@ -174,15 +174,12 @@ static blk_status_t nvme_loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 	if (ret)
 		return ret;
 
+	blk_mq_start_request(req);
 	iod->cmd.common.flags |= NVME_CMD_SGL_METABUF;
 	iod->req.port = nvmet_loop_port;
 	if (!nvmet_req_init(&iod->req, &queue->nvme_cq,
-			&queue->nvme_sq, &nvme_loop_ops)) {
-		nvme_cleanup_cmd(req);
-		blk_mq_start_request(req);
-		nvme_loop_queue_response(&iod->req);
+			&queue->nvme_sq, &nvme_loop_ops))
 		return BLK_STS_OK;
-	}
 
 	if (blk_rq_payload_bytes(req)) {
 		iod->sg_table.sgl = iod->first_sgl;
@@ -196,8 +193,6 @@ static blk_status_t nvme_loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 		iod->req.transfer_len = blk_rq_payload_bytes(req);
 	}
 
-	blk_mq_start_request(req);
-
 	schedule_work(&iod->work);
 	return BLK_STS_OK;
 }