[PATCH v3 2/5] nbd: convert to use blk_mq_get_rq_by_tag()

[Yu Kuai <yukuai3@xxxxxxxxxx>]

blk_mq_tag_to_rq() can only ensure to return valid request in
following situation:

1) client send request message to server first
submit_bio
...
 blk_mq_get_tag
 ...
 blk_mq_get_driver_tag
 ...
 nbd_queue_rq
  nbd_handle_cmd
   nbd_send_cmd

2) client receive respond message from server
recv_work
 nbd_read_stat
  blk_mq_tag_to_rq

If step 1) is missing, blk_mq_tag_to_rq() will return a stale
request, which might be freed. Thus convert to use
blk_mq_get_rq_by_tag() to make sure the returned request is not
freed. However, there are still some problems if the request is
started, and this will be fixed in later patches.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/block/nbd.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 76983185a9a5..ca54a0736090 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -733,11 +733,10 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 	tag = nbd_handle_to_tag(handle);
 	hwq = blk_mq_unique_tag_to_hwq(tag);
 	if (hwq < nbd->tag_set.nr_hw_queues)
-		req = blk_mq_tag_to_rq(nbd->tag_set.tags[hwq],
-				       blk_mq_unique_tag_to_tag(tag));
-	if (!req || !blk_mq_request_started(req)) {
-		dev_err(disk_to_dev(nbd->disk), "Unexpected reply (%d) %p\n",
-			tag, req);
+		req = blk_mq_get_rq_by_tag(nbd->tag_set.tags[hwq],
+					   blk_mq_unique_tag_to_tag(tag));
+	if (!req) {
+		dev_err(disk_to_dev(nbd->disk), "Unexpected reply %d\n", tag);
 		return ERR_PTR(-ENOENT);
 	}
 	trace_nbd_header_received(req, handle);
@@ -799,6 +798,8 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 	}
 out:
 	trace_nbd_payload_received(req, handle);
+	if (req)
+		blk_mq_put_rq_ref(req);
 	mutex_unlock(&cmd->lock);
 	return ret ? ERR_PTR(ret) : cmd;
 }
-- 
2.31.1