Change some operations touching counts_[] to be atomic.

base/metrics/sample_vector.cc

 // 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.
 
 #include "base/metrics/sample_vector.h"
 
 #include "base/logging.h"
 #include "base/metrics/bucket_ranges.h"
 
 using std::vector;
@@ skipping 14 matching lines @@
 void SampleVector::Accumulate(Sample value, Count count) {
   size_t bucket_index = GetBucketIndex(value);
   subtle::NoBarrier_Store(&counts_[bucket_index],
       subtle::NoBarrier_Load(&counts_[bucket_index]) + count);
   IncreaseSum(count * value);
   IncreaseRedundantCount(count);
 }
 
 Count SampleVector::GetCount(Sample value) const {
   size_t bucket_index = GetBucketIndex(value);
-  return counts_[bucket_index];
+  return subtle::NoBarrier_Load(&counts_[bucket_index]);
 }
 
 Count SampleVector::TotalCount() const {
   Count count = 0;
   for (size_t i = 0; i < counts_.size(); i++) {
-    count += counts_[i];
+    count += subtle::NoBarrier_Load(&counts_[i]);
   }
   return count;
 }
 
 Count SampleVector::GetCountAtIndex(size_t bucket_index) const {
   DCHECK(bucket_index < counts_.size());
-  return counts_[bucket_index];
+  return subtle::NoBarrier_Load(&counts_[bucket_index]);
 }
 
 scoped_ptr<SampleCountIterator> SampleVector::Iterator() const {
   return scoped_ptr<SampleCountIterator>(
       new SampleVectorIterator(&counts_, bucket_ranges_));
 }
 
 bool SampleVector::AddSubtractImpl(SampleCountIterator* iter,
                                    HistogramSamples::Operator op) {
   HistogramBase::Sample min;
   HistogramBase::Sample max;
   HistogramBase::Count count;
 
   // Go through the iterator and add the counts into correct bucket.
   size_t index = 0;
   while (index < counts_.size() && !iter->Done()) {
     iter->Get(&min, &max, &count);
     if (min == bucket_ranges_->range(index) &&
         max == bucket_ranges_->range(index + 1)) {
       // Sample matches this bucket!
-      counts_[index] += (op ==  HistogramSamples::ADD) ? count : -count;
+      HistogramBase::Count old_counts =
+          subtle::NoBarrier_Load(&counts_[index]);
+      subtle::NoBarrier_Store(&counts_[index],
+          old_counts + ((op ==  HistogramSamples::ADD) ? count : -count));
       iter->Next();
     } else if (min > bucket_ranges_->range(index)) {
       // Sample is larger than current bucket range. Try next.
       index++;
     } else {
       // Sample is smaller than current bucket range. We scan buckets from
       // smallest to largest, so the sample value must be invalid.
       return false;
     }
   }
@@ skipping 51 matching lines @@
 
 void SampleVectorIterator::Get(HistogramBase::Sample* min,
                                HistogramBase::Sample* max,
                                HistogramBase::Count* count) const {
   DCHECK(!Done());
   if (min != NULL)
     *min = bucket_ranges_->range(index_);
   if (max != NULL)
     *max = bucket_ranges_->range(index_ + 1);
   if (count != NULL)
-    *count = (*counts_)[index_];
+    *count = subtle::NoBarrier_Load(&(*counts_)[index_]);
 }
 
 bool SampleVectorIterator::GetBucketIndex(size_t* index) const {
   DCHECK(!Done());
   if (index != NULL)
     *index = index_;
   return true;
 }
 
 void SampleVectorIterator::SkipEmptyBuckets() {
   if (Done())
     return;
 
   while (index_ < counts_->size()) {
-    if ((*counts_)[index_] != 0)
+    if (subtle::NoBarrier_Load(&(*counts_)[index_]) != 0)
       return;
     index_++;
   }
 }
 
 }  // namespace base