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

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index aed51bf839c41dacfca96c85bd0785c26fa73e6d..0fc8cc66b2d8a9a13713332a51e1e5db831520ec 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -726,7 +726,7 @@ void FieldTrialList::GetInitiallyActiveFieldTrials(
 
     StringPiece trial_name;
     StringPiece group_name;
-    if (entry->activated &&
+    if (subtle::NoBarrier_Load(&entry->activated) &&
         entry->GetTrialAndGroupName(&trial_name, &group_name)) {
       FieldTrial::ActiveGroup group;
       group.trial_name = trial_name.as_string();
@@ -1083,6 +1083,9 @@ void FieldTrialList::ClearParamsFromSharedMemoryForTesting() {
     FieldTrial::FieldTrialEntry* new_entry =
         allocator->GetAsObject<FieldTrial::FieldTrialEntry>(new_ref,
                                                             kFieldTrialType);
+    // Note: No need to use NoBarrier ops below because this is the browser
+    // process so the value we're reading won't change underneath us and the new
+    // entry we're assigning to is not iterable yet.
     new_entry->activated = prev_entry->activated;

[bcwhite, 2016/12/13 13:38:50]

These need to be accessed using atomic ops or you're likely to get a "linter" or
"sanitizer" complaining about non-atomic operations on an atomic variable,
either now or in the future.

[Alexei Svitkine (slow), 2016/12/13 15:26:04]

Done.

     new_entry->pickle_size = pickle.size();
 
@@ -1186,7 +1189,7 @@ bool FieldTrialList::CreateTrialsFromSharedMemory(
         CreateFieldTrial(trial_name.as_string(), group_name.as_string());
 
     trial->ref_ = ref;
-    if (entry->activated) {
+    if (subtle::NoBarrier_Load(&entry->activated)) {
       // Call |group()| to mark the trial as "used" and notify observers, if
       // any. This is useful to ensure that field trials created in child
       // processes are properly reported in crash reports.
@@ -1278,6 +1281,7 @@ void FieldTrialList::AddToAllocatorWhileLocked(
 
   FieldTrial::FieldTrialEntry* entry =
       allocator->GetAsObject<FieldTrial::FieldTrialEntry>(ref, kFieldTrialType);
+  // Note: The entry isn't iterable yet, so no need to use NoBarrier_Store().
   entry->activated = trial_state.activated;

[bcwhite, 2016/12/13 13:38:50]

Still need to so "linters" and "sanitizers" won't complain.

[Alexei Svitkine (slow), 2016/12/13 15:26:04]

Done.

   entry->pickle_size = pickle.size();
 
@@ -1313,7 +1317,7 @@ void FieldTrialList::ActivateFieldTrialEntryWhileLocked(
     FieldTrial::FieldTrialEntry* entry =
         allocator->GetAsObject<FieldTrial::FieldTrialEntry>(ref,
                                                             kFieldTrialType);
-    entry->activated = true;
+    subtle::NoBarrier_Store(&entry->activated, 1);
   }
 }