[runtime] Make access to FLAG_runtime_stats atomic.

Background tasks read this flag, which creates a data race. This patch
works around the data races by making the access to the flag atomic.

The actual fix will be to not mutate the flag.

Bug: chromium:794911
Change-Id: Idcf03b7a1037e876036918418ce989b420784428
Reviewed-on: https://chromium-review.googlesource.com/834508Reviewed-by: Fadi Meawad <fmeawad@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50215}

src/counters-inl.h

@@ -15,7 +15,7 @@ void RuntimeCallTimer::Start(RuntimeCallCounter* counter,
   DCHECK(!IsStarted());
   counter_ = counter;
   parent_.SetValue(parent);
-  if (FLAG_runtime_stats ==
+  if (base::AsAtomic32::Relaxed_Load(&FLAG_runtime_stats) ==
       v8::tracing::TracingCategoryObserver::ENABLED_BY_SAMPLING) {
     return;
   }

src/heap/gc-tracer.cc

@@ -6,6 +6,7 @@
 
 #include <cstdarg>
 
+#include "src/base/atomic-utils.h"
 #include "src/counters.h"
 #include "src/heap/heap-inl.h"
 #include "src/isolate.h"
@@ -52,7 +53,7 @@ GCTracer::BackgroundScope::BackgroundScope(GCTracer* tracer, ScopeId scope)
     : tracer_(tracer), scope_(scope), runtime_stats_enabled_(false) {
   start_time_ = tracer_->heap_->MonotonicallyIncreasingTimeInMs();
   // TODO(cbruni): remove once we fully moved to a trace-based system.
-  if (V8_LIKELY(!FLAG_runtime_stats)) return;
+  if (V8_LIKELY(!base::AsAtomic32::Relaxed_Load(&FLAG_runtime_stats))) return;
   timer_.Start(&counter_, nullptr);
   runtime_stats_enabled_ = true;
 }

src/tracing/tracing-category-observer.cc

@@ -4,6 +4,7 @@
 
 #include "src/tracing/tracing-category-observer.h"
 
+#include "src/base/atomic-utils.h"
 #include "src/flags.h"
 #include "src/tracing/trace-event.h"
 #include "src/v8.h"
@@ -37,12 +38,16 @@ void TracingCategoryObserver::OnTraceEnabled() {
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(
       TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats"), &enabled);
   if (enabled) {
-    v8::internal::FLAG_runtime_stats |= ENABLED_BY_TRACING;
+    base::AsAtomic32::Relaxed_Store(
+        &v8::internal::FLAG_runtime_stats,
+        (v8::internal::FLAG_runtime_stats | ENABLED_BY_TRACING));
   }
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(
       TRACE_DISABLED_BY_DEFAULT("v8.runtime_stats_sampling"), &enabled);
   if (enabled) {
-    v8::internal::FLAG_runtime_stats |= ENABLED_BY_SAMPLING;
+    base::AsAtomic32::Relaxed_Store(
+        &v8::internal::FLAG_runtime_stats,
+        v8::internal::FLAG_runtime_stats | ENABLED_BY_SAMPLING);
   }
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(TRACE_DISABLED_BY_DEFAULT("v8.gc_stats"),
                                      &enabled);
@@ -57,8 +62,10 @@ void TracingCategoryObserver::OnTraceEnabled() {
 }
 
 void TracingCategoryObserver::OnTraceDisabled() {
-  v8::internal::FLAG_runtime_stats &=
-      ~(ENABLED_BY_TRACING | ENABLED_BY_SAMPLING);
+  base::AsAtomic32::Relaxed_Store(
+      &v8::internal::FLAG_runtime_stats,
+      v8::internal::FLAG_runtime_stats &
+          ~(ENABLED_BY_TRACING | ENABLED_BY_SAMPLING));
   v8::internal::FLAG_gc_stats &= ~ENABLED_BY_TRACING;
   v8::internal::FLAG_ic_stats &= ~ENABLED_BY_TRACING;
 }

test/unittests/counters-unittest.cc

@@ -4,6 +4,7 @@
 
 #include <vector>
 
+#include "src/base/atomic-utils.h"
 #include "src/base/platform/time.h"
 #include "src/counters-inl.h"
 #include "src/counters.h"
@@ -55,8 +56,9 @@ static base::TimeTicks RuntimeCallStatsTestNow() {
 class RuntimeCallStatsTest : public TestWithNativeContext {
  public:
   RuntimeCallStatsTest() {
-    FLAG_runtime_stats =
-        v8::tracing::TracingCategoryObserver::ENABLED_BY_NATIVE;
+    base::AsAtomic32::Relaxed_Store(
+        &FLAG_runtime_stats,
+        v8::tracing::TracingCategoryObserver::ENABLED_BY_NATIVE);
     // We need to set {time_} to a non-zero value since it would otherwise
     // cause runtime call timers to think they are uninitialized.
     Sleep(1);
@@ -67,7 +69,7 @@ class RuntimeCallStatsTest : public TestWithNativeContext {
     // Disable RuntimeCallStats before tearing down the isolate to prevent
     // printing the tests table. Comment the following line for debugging
     // purposes.
-    FLAG_runtime_stats = 0;
+    base::AsAtomic32::Relaxed_Store(&FLAG_runtime_stats, 0);
   }
 
   static void SetUpTestCase() {