Avoid unnecessary data race on FLAG_track_double_fields

src/bootstrapper.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/bootstrapper.h"
 
 #include "src/accessors.h"
 #include "src/code-stubs.h"
 #include "src/extensions/externalize-string-extension.h"
 #include "src/extensions/free-buffer-extension.h"
@@ skipping 2572 matching lines @@
   native_context()->set_sloppy_function_map(
       *sloppy_function_map_writable_prototype_);
   native_context()->set_strict_function_map(
       *strict_function_map_writable_prototype_);
 }
 
 
 class NoTrackDoubleFieldsForSerializerScope {
  public:
   explicit NoTrackDoubleFieldsForSerializerScope(Isolate* isolate)
-      : flag_(FLAG_track_double_fields) {
+      : flag_(FLAG_track_double_fields), enabled_(false) {
     if (isolate->serializer_enabled()) {
       // Disable tracking double fields because heap numbers treated as
       // immutable by the serializer.
       FLAG_track_double_fields = false;
+      enabled_ = true;
     }
   }
 
   ~NoTrackDoubleFieldsForSerializerScope() {
-    FLAG_track_double_fields = flag_;
+    if (enabled_) {
+      FLAG_track_double_fields = flag_;
+    }
   }
 
  private:
   bool flag_;
+  bool enabled_;
 };
 
 
 Genesis::Genesis(Isolate* isolate,
                  MaybeHandle<JSGlobalProxy> maybe_global_proxy,
                  v8::Handle<v8::ObjectTemplate> global_proxy_template,
                  v8::ExtensionConfiguration* extensions)
     : isolate_(isolate),
       active_(isolate->bootstrapper()) {
   NoTrackDoubleFieldsForSerializerScope disable_scope(isolate);
@@ skipping 118 matching lines @@
   return from + sizeof(NestingCounterType);
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   DCHECK(!IsActive());
 }
 
 } }  // namespace v8::internal