diff --git a/src/github.com/matrix-org/dendrite/roomserver/types/types.go b/src/github.com/matrix-org/dendrite/roomserver/types/types.go
index ad1000ac4..bdc85bf9b 100644
--- a/src/github.com/matrix-org/dendrite/roomserver/types/types.go
+++ b/src/github.com/matrix-org/dendrite/roomserver/types/types.go
@@ -127,6 +127,8 @@ type StateEntryList struct {
 }
 
 // A RoomRecentEventsUpdater is used to update the recent events in a room.
+// (On postgresql this wraps a database transaction that holds a "FOR UPDATE"
+//  lock on the a row holding the latest events for the room.)
 type RoomRecentEventsUpdater interface {
 	// Store the previous events referenced by an event.
 	StorePreviousEvents(eventNID EventNID, previousEventReferences []gomatrixserverlib.EventReference) error