NoBarrier_Load()/NoBarrier_Store() for manipulating activated field.

base/metrics/field_trial.h

 // Copyright (c) 2012 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.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 45 matching lines @@
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <map>
 #include <memory>
 #include <set>
 #include <string>
 #include <vector>
 
+#include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
 #include "base/files/file.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/shared_memory.h"
 #include "base/metrics/persistent_memory_allocator.h"
 #include "base/observer_list_threadsafe.h"
@@ skipping 60 matching lines @@
 
   // We create one FieldTrialEntry per field trial in shared memory, via
   // AddToAllocatorWhileLocked. The FieldTrialEntry is followed by a
   // base::Pickle object that we unpickle and read from. Any changes to this
   // structure requires a bump in kFieldTrialType id defined in the .cc file.
   struct BASE_EXPORT FieldTrialEntry {
     // Expected size for 32/64-bit check.
     static constexpr size_t kExpectedInstanceSize = 8;
 
     // Whether or not this field trial is activated. This is really just a
-    // boolean but marked as a uint32_t for portability reasons.
-    uint32_t activated;
+    // boolean but using a 32 bit value for portability reasons. It should be
+    // accessed via NoBarrier_Load()/NoBarrier_Store() to prevent the compiler
+    // from doing unexpected optimizations because it thinks that only one
+    // thread is accessing the memory location.
+    subtle::Atomic32 activated;
 
     // Size of the pickled structure, NOT the total size of this entry.
     uint32_t pickle_size;
 
     // Calling this is only valid when the entry is initialized. That is, it
     // resides in shared memory and has a pickle containing the trial name and
     // group name following it.
     bool GetTrialAndGroupName(StringPiece* trial_name,
                               StringPiece* group_name) const;
 
@@ skipping 568 matching lines @@
 
   // Tracks whether CreateTrialsFromCommandLine() has been called.
   bool create_trials_from_command_line_called_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_