diff --git a/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go b/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go
index aaa56e700..b3753b7d7 100644
--- a/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go
+++ b/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go
@@ -35,9 +35,8 @@ type Notifier struct {
 	// for concurrent reads on /sync requests
 	currPos      types.StreamPosition
 	currPosMutex *sync.RWMutex
-	// A map of RoomID => Set<UserID> : Map access is guarded by roomIDToJoinedUsersMutex.
-	roomIDToJoinedUsers      map[string]set
-	roomIDToJoinedUsersMutex *sync.Mutex
+	// A map of RoomID => Set<UserID>
+	roomIDToJoinedUsers map[string]set
 	// A map of user_id => UserStream which can be used to wake a given user's /sync request.
 	// Map access is guarded by userStreamsMutex.
 	userStreams      map[string]*UserStream
@@ -49,12 +48,11 @@ type Notifier struct {
 // the joined users within each of them by calling Notifier.LoadFromDatabase().
 func NewNotifier(pos types.StreamPosition) *Notifier {
 	return &Notifier{
-		currPos:                  pos,
-		currPosMutex:             &sync.RWMutex{},
-		roomIDToJoinedUsers:      make(map[string]set),
-		roomIDToJoinedUsersMutex: &sync.Mutex{},
-		userStreams:              make(map[string]*UserStream),
-		userStreamsMutex:         &sync.Mutex{},
+		currPos:             pos,
+		currPosMutex:        &sync.RWMutex{},
+		roomIDToJoinedUsers: make(map[string]set),
+		userStreams:         make(map[string]*UserStream),
+		userStreamsMutex:    &sync.Mutex{},
 	}
 }
 
@@ -88,7 +86,7 @@ func (n *Notifier) OnNewEvent(ev *gomatrixserverlib.Event, pos types.StreamPosit
 			case "leave":
 				fallthrough
 			case "ban":
-				n.userLeft(ev.RoomID(), userID)
+				n.removeJoinedUser(ev.RoomID(), userID)
 			}
 		}
 	}
@@ -139,9 +137,7 @@ func (n *Notifier) Load(db *storage.SyncServerDatabase) error {
 // these rooms will wake the given users /sync requests. This should be called prior to ANY calls to
 // OnNewEvent (eg on startup) to prevent racing.
 func (n *Notifier) setUsersJoinedToRooms(roomIDToUserIDs map[string][]string) {
-	// This is just the bulk form of userJoined where we only lock once.
-	n.roomIDToJoinedUsersMutex.Lock()
-	defer n.roomIDToJoinedUsersMutex.Unlock()
+	// This is just the bulk form of addJoinedUser
 	for roomID, userIDs := range roomIDToUserIDs {
 		if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
 			n.roomIDToJoinedUsers[roomID] = make(set)
@@ -180,27 +176,24 @@ func (n *Notifier) fetchUserStream(userID string, makeIfNotExists bool) *UserStr
 	return stream
 }
 
+// Not thread-safe: must be called on the OnNewEvent goroutine only
 func (n *Notifier) addJoinedUser(roomID, userID string) {
-	n.roomIDToJoinedUsersMutex.Lock()
-	defer n.roomIDToJoinedUsersMutex.Unlock()
 	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
 		n.roomIDToJoinedUsers[roomID] = make(set)
 	}
 	n.roomIDToJoinedUsers[roomID].add(userID)
 }
 
-func (n *Notifier) userLeft(roomID, userID string) {
-	n.roomIDToJoinedUsersMutex.Lock()
-	defer n.roomIDToJoinedUsersMutex.Unlock()
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) removeJoinedUser(roomID, userID string) {
 	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
 		n.roomIDToJoinedUsers[roomID] = make(set)
 	}
 	n.roomIDToJoinedUsers[roomID].remove(userID)
 }
 
+// Not thread-safe: must be called on the OnNewEvent goroutine only
 func (n *Notifier) joinedUsers(roomID string) (userIDs []string) {
-	n.roomIDToJoinedUsersMutex.Lock()
-	defer n.roomIDToJoinedUsersMutex.Unlock()
 	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
 		return
 	}
@@ -218,11 +211,6 @@ func (s set) remove(str string) {
 	delete(s, str)
 }
 
-func (s set) has(str string) bool {
-	_, ok := s[str]
-	return ok
-}
-
 func (s set) values() (vals []string) {
 	for str := range s {
 		vals = append(vals, str)