3 files changed, 12 insertions, 19 deletions
diff --git a/courses.go b/courses.go
index ccb5313..45e6fd9 100644
--- a/courses.go
+++ b/courses.go
@@ -53,8 +53,7 @@ type courseT struct {
 	Group        courseGroupT
 	Teacher      string
 	Location     string
-	Usems        map[string](*usemT)
-	UsemsLock    sync.RWMutex
+	Usems        sync.Map /* string, *usemT */
 }
 
 const (
@@ -145,9 +144,7 @@ func setupCourses() error {
 			}
 			break
 		}
-		currentCourse := courseT{
-			Usems: make(map[string]*usemT),
-		} //exhaustruct:ignore
+		currentCourse := courseT{} //exhaustruct:ignore
 		err = rows.Scan(
 			&currentCourse.ID,
 			&currentCourse.Max,
diff --git a/wsc.go b/wsc.go
index 4142512..ffcb6da 100644
--- a/wsc.go
+++ b/wsc.go
@@ -93,11 +93,7 @@ func handleConn(
 		for courseID, course := range courses {
 			usem := &usemT{} //exhaustruct:ignore
 			usem.init()
-			func() {
-				course.UsemsLock.Lock()
-				defer course.UsemsLock.Unlock()
-				course.Usems[userID] = usem
-			}()
+			course.Usems.Store(userID, usem)
 			usems[courseID] = usem
 		}
 	}()
@@ -105,11 +101,7 @@ func handleConn(
 		coursesLock.RLock()
 		defer coursesLock.RUnlock()
 		for _, course := range courses {
-			func() {
-				course.UsemsLock.Lock()
-				defer course.UsemsLock.Unlock()
-				delete(course.Usems, userID)
-			}()
+			course.Usems.Delete(userID)
 		}
 		atomic.AddInt64(&usemCount, -int64(len(courses)))
 	}()
diff --git a/wsp.go b/wsp.go
index 1c30d4a..125608b 100644
--- a/wsp.go
+++ b/wsp.go
@@ -82,11 +82,15 @@ func makeReportError(ctx context.Context, conn *websocket.Conn) reportErrorT {
 
 func propagateSelectedUpdate(courseID int) {
 	course := courses[courseID]
-	course.UsemsLock.RLock()
-	defer course.UsemsLock.RUnlock()
-	for _, usem := range course.Usems {
+	course.Usems.Range(func(key, value interface{}) bool {
+		_ = key
+		usem, ok := value.(*usemT)
+		if !ok {
+			panic("Usems contains non-\"*usemT\" value")
+		}
 		usem.set()
-	}
+		return true
+	})
 }
 
 func sendSelectedUpdate(