Annotate the benign race in ScriptForbiddenScope::isScriptForbidden().

third_party/WebKit/Source/platform/ScriptForbiddenScope.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ScriptForbiddenScope_h
 #define ScriptForbiddenScope_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Allocator.h"
 #include "wtf/AutoReset.h"
+#include "wtf/DynamicAnnotations.h"
 #include "wtf/Optional.h"
 
 namespace blink {
 
 // Scoped disabling of script execution on the main thread,
 // and only to be used by the main thread.
 class PLATFORM_EXPORT ScriptForbiddenScope final {
   STACK_ALLOCATED();
   WTF_MAKE_NONCOPYABLE(ScriptForbiddenScope);
 
@@ skipping 24 matching lines @@
   }
   static void exit() {
     DCHECK(isMainThread());
     DCHECK(s_scriptForbiddenCount);
     --s_scriptForbiddenCount;
   }
   static bool isScriptForbidden() {
     // Check s_scriptForbiddenCount first to avoid any calls to isMainThread()
     // since under normal situations where we check this (ex. inside
     // V8ScriptRunner) the value should always be zero.
+    WTF_ANNOTATE_BENIGN_RACE(&s_scriptForbiddenCount,

[Alexander Potapenko, 2017/03/17 10:14:50]

Please no.
WTF_ANNOTATE_BENIGN_RACE is a beast we need to get rid of, not let it grow.
If you're reading something from several threads, these reads must be
synchronized.

+                             "Safe to racy read s_scriptForbiddenCount since "
+                             "the value only matters on the main thread.");
     return s_scriptForbiddenCount && isMainThread();

[Alexander Potapenko, 2017/03/17 10:14:50]

If the value only matters on the main thread, how about we read it on the main
thread only?
Putting isMainThread() on the first place should solve the problem:

  return isMainThread() && s_scriptForbiddenCount;

If it does not, someone might be modifying s_scriptForbiddenCount from another
thread.

   }
 
  private:
   static unsigned s_scriptForbiddenCount;
 };
 
 // Scoped disabling of script execution on the main thread,
 // if called on the main thread.
 //
 // No effect when used by from other threads -- simplifies
@@ skipping 13 matching lines @@
   ~ScriptForbiddenIfMainThreadScope() {
     if (m_IsMainThread)
       ScriptForbiddenScope::exit();
   }
   bool m_IsMainThread;
 };
 
 }  // namespace blink
 
 #endif  // ScriptForbiddenScope_h