diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index 94106141..97425f52 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -620,18 +620,13 @@ func checkAllowedByState(e *gomatrixserverlib.Event, stateEvents []*gomatrixserv
 	return gomatrixserverlib.Allowed(e, &authUsingState)
 }
 
-var processEventWithMissingStateMutexes = map[string]*sync.Mutex{}
+var processEventWithMissingStateMutexes = &internal.MutexByRoom{}
 
 func (t *txnReq) processEventWithMissingState(
 	ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion,
 ) error {
-	mu, ok := processEventWithMissingStateMutexes[e.RoomID()]
-	if !ok {
-		processEventWithMissingStateMutexes[e.RoomID()] = &sync.Mutex{}
-		mu = processEventWithMissingStateMutexes[e.RoomID()]
-	}
-	mu.Lock()
-	defer mu.Unlock()
+	processEventWithMissingStateMutexes.Lock(e.RoomID())
+	defer processEventWithMissingStateMutexes.Unlock(e.RoomID())
 
 	// Do this with a fresh context, so that we keep working even if the
 	// original request times out. With any luck, by the time the remote