sadly, serial is not acceptable here

tasks/persistent_cloud.yml

@@ -73,12 +73,9 @@
   ignore_errors: True
   register: hostkey
   when: host_is_up|failed
-#
-# Set serial: 1 here due to locking bug with known_hosts
-#
+
 - name: add new ssh host key (you still need to add it to official ssh_host_keys later)
   local_action: known_hosts path={{ item }} key={{ hostkey.stdout }} host={{ inventory_hostname }} state=present
-  serial: 1
   ignore_errors: True
   with_items:
   - /root/.ssh/known_hosts