diff --git a/flowcontrol/flow_control_manager.go b/flowcontrol/flow_control_manager.go
index db62009b..a48c8b13 100644
--- a/flowcontrol/flow_control_manager.go
+++ b/flowcontrol/flow_control_manager.go
@@ -138,6 +138,7 @@ func (f *flowControlManager) AddBytesRead(streamID protocol.StreamID, n protocol
 func (f *flowControlManager) GetWindowUpdates() (res []WindowUpdate) {
 	f.mutex.Lock()
 	defer f.mutex.Unlock()
+
 	connFlowController := f.streamFlowController[0]
 
 	// get WindowUpdates for streams
@@ -161,8 +162,9 @@ func (f *flowControlManager) GetWindowUpdates() (res []WindowUpdate) {
 }
 
 func (f *flowControlManager) GetReceiveWindow(streamID protocol.StreamID) (protocol.ByteCount, error) {
-	f.mutex.Lock()
-	defer f.mutex.Unlock()
+	f.mutex.RLock()
+	defer f.mutex.RUnlock()
+
 	flowController, err := f.getFlowController(streamID)
 	if err != nil {
 		return 0, err
@@ -172,10 +174,10 @@ func (f *flowControlManager) GetReceiveWindow(streamID protocol.StreamID) (proto
 
 // streamID must not be 0 here
 func (f *flowControlManager) AddBytesSent(streamID protocol.StreamID, n protocol.ByteCount) error {
-	// Only lock the part reading from the map, since send-windows are only accessed from the session goroutine.
 	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
 	fc, err := f.getFlowController(streamID)
-	f.mutex.Unlock()
 	if err != nil {
 		return err
 	}
@@ -190,10 +192,10 @@ func (f *flowControlManager) AddBytesSent(streamID protocol.StreamID, n protocol
 
 // must not be called with StreamID 0
 func (f *flowControlManager) SendWindowSize(streamID protocol.StreamID) (protocol.ByteCount, error) {
-	// Only lock the part reading from the map, since send-windows are only accessed from the session goroutine.
 	f.mutex.RLock()
+	defer f.mutex.RUnlock()
+
 	fc, err := f.getFlowController(streamID)
-	f.mutex.RUnlock()
 	if err != nil {
 		return 0, err
 	}
@@ -207,24 +209,23 @@ func (f *flowControlManager) SendWindowSize(streamID protocol.StreamID) (protoco
 }
 
 func (f *flowControlManager) RemainingConnectionWindowSize() protocol.ByteCount {
-	// Only lock the part reading from the map, since send-windows are only accessed from the session goroutine.
 	f.mutex.RLock()
-	res := f.streamFlowController[0].SendWindowSize()
-	f.mutex.RUnlock()
-	return res
+	defer f.mutex.RUnlock()
+
+	return f.streamFlowController[0].SendWindowSize()
 }
 
 // streamID may be 0 here
 func (f *flowControlManager) UpdateWindow(streamID protocol.StreamID, offset protocol.ByteCount) (bool, error) {
-	// Only lock the part reading from the map, since send-windows are only accessed from the session goroutine.
 	f.mutex.Lock()
-	streamFlowController, err := f.getFlowController(streamID)
-	f.mutex.Unlock()
+	defer f.mutex.Unlock()
+
+	fc, err := f.getFlowController(streamID)
 	if err != nil {
 		return false, err
 	}
 
-	return streamFlowController.UpdateSendWindow(offset), nil
+	return fc.UpdateSendWindow(offset), nil
 }
 
 func (f *flowControlManager) getFlowController(streamID protocol.StreamID) (*flowController, error) {