Create a thread safe version of StatsCounters and use.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 3154f72ac3d303d7c423c3eded8f2cfd6f993606..3ea27298024334d7c15c16058d99d8d9160bbffb 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2549,7 +2549,6 @@ Isolate::~Isolate() {
   delete logger_;
   logger_ = NULL;
 
-  delete counters_;
   counters_ = NULL;
 
   delete handle_scope_implementer_;
@@ -2635,14 +2634,22 @@ bool Isolate::PropagatePendingExceptionToExternalTryCatch() {
   return true;
 }
 
+static base::LazyMutex initialize_counters_mutex = LAZY_MUTEX_INITIALIZER;
+
+bool Isolate::InitializeCounters() {
+  if (counters_ != nullptr) return false;

[Clemens Hammacher, 2017/06/02 13:08:49]

This pattern is not safe in general. A thread observing that {counters_ !=
nullptr} is not guaranteed to also observe all other updates performed inside
the locked scope below. This would at least require atomic updates of the
{counters_} field, or explicit memory barries.

Overall, I don't see why we need the mutex here in the first place. Is it
allowed to call this function from a background task?

[kschimpf, 2017/06/02 15:01:02]

I was probably too concerned about this, since the way code was previously
written, it could be called on background threads. However, I agree that we
probably could have presumed this. Now that the StatsTable of an Isolate has
been moved to a field in the Counters class, we can make stronger assumptions
(by not making this method public). Will fix as part of CL
https://codereview.chromium.org/2918703002

[kschimpf, 2017/06/02 17:06:27]

Fixed in cl https://codereview.chromium.org/2919953003

+  base::LockGuard<base::Mutex> guard(initialize_counters_mutex.Pointer());
+  if (counters_ != nullptr) return false;
+  counters_shared_ = std::make_shared<Counters>(this);
+  counters_ = counters_shared_.get();
+  return true;
+}
 
 void Isolate::InitializeLoggingAndCounters() {
   if (logger_ == NULL) {
     logger_ = new Logger(this);
   }
-  if (counters_ == NULL) {
-    counters_ = new Counters(this);
-  }
+  InitializeCounters();
 }