Use lock-free lazy initialization for static histogram references

net/disk_cache/stats_histogram.cc

 // Copyright (c) 2011 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 "net/disk_cache/stats_histogram.h"
 
 #include "base/logging.h"
 #include "net/disk_cache/stats.h"
 
 namespace disk_cache {
 
 using base::Histogram;
 using base::StatisticsRecorder;
 
 // Static.
 const Stats* StatsHistogram::stats_ = NULL;
 
 StatsHistogram::~StatsHistogram() {
   // Only cleanup what we set.
   if (init_)
     stats_ = NULL;
 }
 
-scoped_refptr<StatsHistogram> StatsHistogram::StatsHistogramFactoryGet(
+StatsHistogram* StatsHistogram::StatsHistogramFactoryGet(
     const std::string& name) {
-  scoped_refptr<Histogram> histogram(NULL);
+  Histogram* histogram(NULL);
 
   Sample minimum = 1;
   Sample maximum = disk_cache::Stats::kDataSizesLength - 1;
   size_t bucket_count = disk_cache::Stats::kDataSizesLength;
 
   if (StatisticsRecorder::FindHistogram(name, &histogram)) {
-    DCHECK(histogram.get() != NULL);
+    DCHECK(histogram != NULL);
   } else {
-    StatsHistogram* stats_histogram = new StatsHistogram(name, minimum, maximum,
-                                                         bucket_count);
+    // To avoid racy destruction at shutdown, the following will be leaked.
+    StatsHistogram* stats_histogram =
+        new StatsHistogram(name, minimum, maximum, bucket_count);
     stats_histogram->InitializeBucketRange();
-    histogram = stats_histogram;
-    histogram->SetFlags(kUmaTargetedHistogramFlag);
-    StatisticsRecorder::RegisterOrDiscardDuplicate(&histogram);
+    stats_histogram->SetFlags(kUmaTargetedHistogramFlag);
+    histogram = StatisticsRecorder::RegisterOrDeleteDuplicate(stats_histogram);
   }
 
   DCHECK(HISTOGRAM == histogram->histogram_type());
   DCHECK(histogram->HasConstructorArguments(minimum, maximum, bucket_count));
 
   // We're preparing for an otherwise unsafe upcast by ensuring we have the
   // proper class type.
-  Histogram* temp_histogram = histogram.get();
-  StatsHistogram* temp_stats_histogram =
-      static_cast<StatsHistogram*>(temp_histogram);
+  StatsHistogram* return_histogram = static_cast<StatsHistogram*>(histogram);
   // Validate upcast by seeing that we're probably providing the checksum.
-  CHECK_EQ(temp_stats_histogram->StatsHistogram::CalculateRangeChecksum(),
-           temp_stats_histogram->CalculateRangeChecksum());
-  scoped_refptr<StatsHistogram> return_histogram(temp_stats_histogram);
+  CHECK_EQ(return_histogram->StatsHistogram::CalculateRangeChecksum(),
+           return_histogram->CalculateRangeChecksum());
   return return_histogram;
 }
 
 bool StatsHistogram::Init(const Stats* stats) {
   DCHECK(stats);
   if (stats_)
     return false;
 
   // We support statistics report for only one cache.
   init_ = true;
@@ skipping 27 matching lines @@
   return NO_INCONSISTENCIES;  // This class won't monitor inconsistencies.
 }
 
 uint32 StatsHistogram::CalculateRangeChecksum() const {
   // We don't calculate checksums, so at least establish a unique constant.
   const uint32 kStatsHistogramChecksum = 0x0cecce;
   return kStatsHistogramChecksum;
 }
 
 }  // namespace disk_cache