Use subtle::NoBarrier* statements for Histogram flags_ field.

base/metrics/histogram_base.cc

Index: base/metrics/histogram_base.cc
diff --git a/base/metrics/histogram_base.cc b/base/metrics/histogram_base.cc
index 6b3f69c2c047a63198d6cf69e94ec6e29336e8b7..92da79f165f465da0fd2a98041fe1bea6f933896 100644
--- a/base/metrics/histogram_base.cc
+++ b/base/metrics/histogram_base.cc
@@ -72,11 +72,13 @@ void HistogramBase::CheckName(const StringPiece& name) const {
 }
 
 void HistogramBase::SetFlags(int32 flags) {
-  flags_ |= flags;
+  HistogramBase::Count old_flags = subtle::NoBarrier_Load(&flags_);
+  subtle::NoBarrier_Store(&flags_, old_flags | flags);

[Alexander Potapenko, 2015/11/18 11:23:28]

The question is whether there is any data that is being concurrently read or
written together with the flags.
For example, if there's a data pointer that's written by one thread before
SetFlags(kSomeFlag) and dereferenced by another thread after checking flags() &
kSomeFlag, there must be a barrier between the accesses to flags_ and to the
pointer (i.e. accesses to flags_ must have acquire/release semantics).
If your flags won't associated data it's fine to keep the accesses nobarrier.

[Alexei Svitkine (slow), 2015/11/18 16:37:12]

I don't think there's a case where there is associated data that's not already
guarded by a lock.

The only case where there is an associate data is the callback flag, but the
callback itself is accessed after grabbing a lock on StatisticsRecorder. The
flag is just used to ensure that 99.999% cases that don't have a callback set
don't need to do any synchronization. (And inconsistencies between the flag and
the callback are safe - i.e. it's not critical if one doesn't match the other
due to caching for some period of time until a barrier happens elsewhere.)

 }
 
 void HistogramBase::ClearFlags(int32 flags) {
-  flags_ &= ~flags;
+  HistogramBase::Count old_flags = subtle::NoBarrier_Load(&flags_);
+  subtle::NoBarrier_Store(&flags_, old_flags & ~flags);
 }
 
 void HistogramBase::AddTime(const TimeDelta& time) {
@@ -119,7 +121,7 @@ void HistogramBase::WriteJSON(std::string* output) const {
 }
 
 void HistogramBase::FindAndRunCallback(HistogramBase::Sample sample) const {
-  if ((flags_ & kCallbackExists) == 0)
+  if ((flags() & kCallbackExists) == 0)
     return;
 
   StatisticsRecorder::OnSampleCallback cb =