sync: fix TestMutexConcurrent test

Accessing the same variable from multiple goroutines is unsafe, and will
fail with parallelism. A lightweight way to avoid issues is by using
atomic variables.

src/sync/mutex_test.go

@@ -58,9 +58,9 @@ func TestMutexUncontended(t *testing.T) {
 // It will fail if multiple goroutines hold the lock simultaneously.
 func TestMutexConcurrent(t *testing.T) {
 	var mu sync.Mutex
-	var active uint
-	var completed uint
-	ok := true
+	var active atomic.Uint32
+	var completed atomic.Uint32
+	var fail atomic.Uint32
 
 	const n = 10
 	for i := 0; i < n; i++ {
@@ -74,11 +74,11 @@ func TestMutexConcurrent(t *testing.T) {
 			mu.Lock()
 
 			// Increment the active counter.
-			active++
+			nowActive := active.Add(1)
 
-			if active > 1 {
+			if nowActive > 1 {
 				// Multiple things are holding the lock at the same time.
-				ok = false
+				fail.Store(1)
 			} else {
 				// Delay a bit.
 				for k := j; k < n; k++ {
@@ -87,10 +87,11 @@ func TestMutexConcurrent(t *testing.T) {
 			}
 
 			// Decrement the active counter.
-			active--
+			var one = 1
+			active.Add(uint32(-one))
 
 			// This is completed.
-			completed++
+			completed.Add(1)
 
 			mu.Unlock()
 		}()
@@ -104,10 +105,10 @@ func TestMutexConcurrent(t *testing.T) {
 
 		// Acquire the lock and check whether everything has completed.
 		mu.Lock()
-		done = completed == n
+		done = completed.Load() == n
 		mu.Unlock()
 	}
-	if !ok {
+	if fail.Load() != 0 {
 		t.Error("lock held concurrently")
 	}
 }