Re: [PATCH v4 2/6] nbd: convert to use blk_mq_find_and_get_req()

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_find_and_get_req() 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 next patch.

Signed-off-by: Yu Kuai <redacted>
---
 drivers/block/nbd.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 5170a630778d..920da390635c 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c

@@ -718,12 +718,13 @@ 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));
+		req = blk_mq_find_and_get_req(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);
-		return ERR_PTR(-ENOENT);
+		ret = -ENOENT;
+		goto put_req;
 	}
 	trace_nbd_header_received(req, handle);
 	cmd = blk_mq_rq_to_pdu(req);

@@ -785,6 +786,9 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 out:
 	trace_nbd_payload_received(req, handle);
 	mutex_unlock(&cmd->lock);
+put_req:
+	if (req)
+		blk_mq_put_rq_ref(req);
 	return ret ? ERR_PTR(ret) : cmd;