Change thread check to race check

webrtc/base/race_checker.cc

 /*
  *  Copyright (c) 2016 The WebRTC 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 in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/base/race_checker.h"
 
 namespace rtc {
 
 RaceChecker::RaceChecker() {}
 
+// Note that the implementation here is in itself racy, but we pretend it does
+// not matter because we want this useful in release builds without having to
+// pay the cost of using atomics. A race hitting the race checker is likely to
+// cause access_count_ to diverge from zero and therefore cause the ThreadRef
+// comparison to fail, signaling a race, although it may not be in the exact
+// spot where a race *first* appeared in the code we're trying to protect. There
+// is also a chance that an actual race is missed, however the probability of
+// that has been considered small enough to be an acceptable trade off.
 bool RaceChecker::Acquire() const {
   const PlatformThreadRef current_thread = CurrentThreadRef();
   // Set new accessing thread if this is a new use.
   if (access_count_++ == 0)
     accessing_thread_ = current_thread;
   // If this is being used concurrently this check will fail for the second
   // thread entering since it won't set the thread. Recursive use of checked
   // methods are OK since the accessing thread remains the same.
   const PlatformThreadRef accessing_thread = accessing_thread_;
   return IsThreadRefEqual(accessing_thread, current_thread);
@@ skipping 10 matching lines @@
 bool RaceCheckerScope::RaceDetected() const {
   return !race_check_ok_;
 }
 
 RaceCheckerScope::~RaceCheckerScope() {
   race_checker_->Release();
 }
 
 }  // namespace internal
 }  // namespace rtc