nvme-tcp: rerun io_work if req_list is not empty

A possible race condition exists where the request to send data is
enqueued from nvme_tcp_handle_r2t()'s will not be observed by
nvme_tcp_send_all() if it happens to be running. The driver relies on
io_work to send the enqueued request when it is runs again, but the
concurrently running nvme_tcp_send_all() may not have released the
send_mutex at that time. If no future commands are enqueued to re-kick
the io_work, the request will timeout in the SEND_H2C state, resulting
in a timeout error like:

  nvme nvme0: queue 1: timeout request 0x3 type 6

Ensure the io_work continues to run as long as the req_list is not empty.

Fixes: db5ad6b7f8 ("nvme-tcp: try to send request in queue_rq context")
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/tcp.c

@@ -1140,7 +1140,8 @@ static void nvme_tcp_io_work(struct work_struct *w)
 				pending = true;
 			else if (unlikely(result < 0))
 				break;
-		}
+		} else
+			pending = !llist_empty(&queue->req_list);
 
 		result = nvme_tcp_try_recv(queue);
 		if (result > 0)