runtime/trace: convert tracing.enabled to atomic type

Updates #53821

Change-Id: I8a063ae94568cd2ea65c2e891618069a96139891
Reviewed-on: https://go-review.googlesource.com/c/go/+/423884
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/runtime/trace/annotation.go

@@ -178,8 +178,7 @@ func (r *Region) End() {
 // The information is advisory only. The tracing status
 // may have changed by the time this function returns.
 func IsEnabled() bool {
-	enabled := atomic.LoadInt32(&tracing.enabled)
-	return enabled == 1
+	return tracing.enabled.Load()
 }
 
 //

src/runtime/trace/trace.go

@@ -134,7 +134,7 @@ func Start(w io.Writer) error {
 			w.Write(data)
 		}
 	}()
-	atomic.StoreInt32(&tracing.enabled, 1)
+	tracing.enabled.Store(true)
 	return nil
 }
 
@@ -143,12 +143,12 @@ func Start(w io.Writer) error {
 func Stop() {
 	tracing.Lock()
 	defer tracing.Unlock()
-	atomic.StoreInt32(&tracing.enabled, 0)
+	tracing.enabled.Store(false)
 
 	runtime.StopTrace()
 }
 
 var tracing struct {
-	sync.Mutex       // gate mutators (Start, Stop)
-	enabled    int32 // accessed via atomic
+	sync.Mutex // gate mutators (Start, Stop)
+	enabled    atomic.Bool
 }