diff --git a/ironic/common/states.py b/ironic/common/states.py
index bb24822f34..a45e1f7b34 100644
--- a/ironic/common/states.py
+++ b/ironic/common/states.py
@@ -216,6 +216,8 @@ UNRESCUEFAIL = 'unrescue failed'
 UNRESCUING = 'unrescuing'
 """ Node is being restored from rescue mode (to active state). """
 
+# NOTE(kaifeng): INSPECTING is allowed to keep backwards compatibility,
+# starting from API 1.39 node update is disallowed in this state.
 UPDATE_ALLOWED_STATES = (DEPLOYFAIL, INSPECTING, INSPECTFAIL, INSPECTWAIT,
                          CLEANFAIL, ERROR, VERIFYING, ADOPTFAIL, RESCUEFAIL,
                          UNRESCUEFAIL)
diff --git a/ironic/conductor/manager.py b/ironic/conductor/manager.py
index d832b1eefe..d79104aa8e 100644
--- a/ironic/conductor/manager.py
+++ b/ironic/conductor/manager.py
@@ -170,6 +170,9 @@ class ConductorManager(base_manager.BaseConductorManager):
 
         # TODO(dtantsur): reconsider allowing changing some (but not all)
         # interfaces for active nodes in the future.
+        # NOTE(kaifeng): INSPECTING is allowed to keep backwards
+        # compatibility, starting from API 1.39 node update is disallowed
+        # in this state.
         allowed_update_states = [states.ENROLL, states.INSPECTING,
                                  states.INSPECTWAIT, states.MANAGEABLE,
                                  states.AVAILABLE]