Avoid threading races on TraceSamplingThread's members.

base/debug/trace_event_impl.cc

Index: base/debug/trace_event_impl.cc
diff --git a/base/debug/trace_event_impl.cc b/base/debug/trace_event_impl.cc
index 4a365a773dc37a6d776eff5785ed1eb8d1ad989e..6a1ad65445ee786c6e19d053a3def3cef70e5203 100644
--- a/base/debug/trace_event_impl.cc
+++ b/base/debug/trace_event_impl.cc
@@ -714,6 +714,7 @@ class TraceSamplingThread : public PlatformThread::Delegate {
                                      const char** category,
                                      const char** name);
   std::vector<TraceBucketData> sample_buckets_;
+  Lock lock_;
   bool thread_running_;
   scoped_ptr<CancellationFlag> cancellation_flag_;
   scoped_ptr<WaitableEvent> waitable_event_for_testing_;
@@ -722,6 +723,7 @@ class TraceSamplingThread : public PlatformThread::Delegate {
 
 TraceSamplingThread::TraceSamplingThread()
     : thread_running_(false) {
+  AutoLock lock(lock_);
   cancellation_flag_.reset(new CancellationFlag);
 }
 
@@ -732,10 +734,14 @@ void TraceSamplingThread::ThreadMain() {
   PlatformThread::SetName("Sampling Thread");
   thread_running_ = true;
   const int kSamplingFrequencyMicroseconds = 1000;
-  while (!cancellation_flag_->IsSet()) {
+  while (true) {
     PlatformThread::Sleep(
         TimeDelta::FromMicroseconds(kSamplingFrequencyMicroseconds));
     GetSamples();
+
+    AutoLock lock(lock_);
+    if (cancellation_flag_->IsSet())
+      break;
     if (waitable_event_for_testing_.get())
       waitable_event_for_testing_->Signal();
   }
@@ -782,11 +788,13 @@ void TraceSamplingThread::ExtractCategoryAndName(const char* combined,
 }
 
 void TraceSamplingThread::Stop() {
+  AutoLock lock(lock_);
   cancellation_flag_->Set();
 }
 
 void TraceSamplingThread::InstallWaitableEventForSamplingTesting(
     WaitableEvent* waitable_event) {
+  AutoLock lock(lock_);
   waitable_event_for_testing_.reset(waitable_event);
 }