diff --git a/httpstaticserver.go b/httpstaticserver.go
index 618bc1b..d3e5b37 100644
--- a/httpstaticserver.go
+++ b/httpstaticserver.go
@@ -42,6 +42,11 @@ type IndexFileItem struct {
 	Info os.FileInfo
 }
 
+type Directory struct {
+	size  map[string]int64
+	mutex *sync.RWMutex
+}
+
 type HTTPStaticServer struct {
 	Root            string
 	Prefix          string
@@ -630,7 +635,7 @@ func (s *HTTPStaticServer) hJSONList(w http.ResponseWriter, r *http.Request) {
 	w.Write(data)
 }
 
-var dirSizeMap = make(map[string]int64)
+var dirInfoSize = Directory{size: make(map[string]int64), mutex: &sync.RWMutex{}}
 
 func (s *HTTPStaticServer) makeIndex() error {
 	var indexes = make([]IndexFileItem, 0)
@@ -650,21 +655,28 @@ func (s *HTTPStaticServer) makeIndex() error {
 		return nil
 	})
 	s.indexes = indexes
-	dirSizeMap = make(map[string]int64)
 	return err
 }
 
 func (s *HTTPStaticServer) historyDirSize(dir string) int64 {
-	var size int64
-	if size, ok := dirSizeMap[dir]; ok {
+	dirInfoSize.mutex.RLock()
+	size, ok := dirInfoSize.size[dir]
+	dirInfoSize.mutex.RUnlock()
+
+	if ok {
 		return size
 	}
+
 	for _, fitem := range s.indexes {
 		if filepath.HasPrefix(fitem.Path, dir) {
 			size += fitem.Info.Size()
 		}
 	}
-	dirSizeMap[dir] = size
+
+	dirInfoSize.mutex.Lock()
+	dirInfoSize.size[dir] = size
+	dirInfoSize.mutex.Unlock()
+
 	return size
 }
 
@@ -729,10 +741,9 @@ func (s *HTTPStaticServer) readAccessConf(realPath string) (ac AccessConf) {
 }
 
 func deepPath(basedir, name string) string {
-	isDir := true
 	// loop max 5, incase of for loop not finished
 	maxDepth := 5
-	for depth := 0; depth <= maxDepth && isDir; depth += 1 {
+	for depth := 0; depth <= maxDepth; depth += 1 {
 		finfos, err := ioutil.ReadDir(filepath.Join(basedir, name))
 		if err != nil || len(finfos) != 1 {
 			break