Use lock-free lazy initialization for static histogram references

net/disk_cache/histogram_macros.h

 // 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.
 
 // This file contains macros to simplify histogram reporting from the disk
 // cache. The main issue is that we want to have separate histograms for each
 // type of cache (regular vs. media, etc), without adding the complexity of
 // keeping track of a potentially large number of histogram objects that have to
 // survive the backend object that created them.
 
 #ifndef NET_DISK_CACHE_HISTOGRAM_MACROS_H_
 #define NET_DISK_CACHE_HISTOGRAM_MACROS_H_
 #pragma once
 
 // -----------------------------------------------------------------------------
 
 // These histograms follow the definition of UMA_HISTOGRAMN_XXX except that
 // whenever the name changes (the experiment group changes), the histrogram
 // object is re-created.
+// Note: These macros are only run on one thread, so the declarations of
+// |counter| was made static (i.e., there will be no race for reinitialization).
 
 #define CACHE_HISTOGRAM_CUSTOM_COUNTS(name, sample, min, max, bucket_count) \
     do { \
-      scoped_refptr<base::Histogram> counter; \
+      static base::Histogram* counter(NULL); \
       if (!counter || name != counter->histogram_name()) \
         counter = base::Histogram::FactoryGet( \
             name, min, max, bucket_count, \
             base::Histogram::kUmaTargetedHistogramFlag); \
       counter->Add(sample); \
     } while (0)
 
 #define CACHE_HISTOGRAM_COUNTS(name, sample) CACHE_HISTOGRAM_CUSTOM_COUNTS( \
     name, sample, 1, 1000000, 50)
 
 #define CACHE_HISTOGRAM_COUNTS_10000(name, sample) \
     CACHE_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 10000, 50)
 
 #define CACHE_HISTOGRAM_COUNTS_50000(name, sample) \
     CACHE_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 50000000, 50)
 
 #define CACHE_HISTOGRAM_CUSTOM_TIMES(name, sample, min, max, bucket_count) \
     do { \
-      scoped_refptr<base::Histogram> counter; \
+      static base::Histogram* counter(NULL); \
       if (!counter || name != counter->histogram_name()) \
         counter = base::Histogram::FactoryTimeGet( \
             name, min, max, bucket_count, \
             base::Histogram::kUmaTargetedHistogramFlag); \
       counter->AddTime(sample); \
     } while (0)
 
 #define CACHE_HISTOGRAM_TIMES(name, sample) CACHE_HISTOGRAM_CUSTOM_TIMES( \
     name, sample, base::TimeDelta::FromMilliseconds(1), \
     base::TimeDelta::FromSeconds(10), 50)
 
 #define CACHE_HISTOGRAM_ENUMERATION(name, sample, boundary_value) do { \
-    scoped_refptr<base::Histogram> counter; \
+    static base::Histogram* counter(NULL); \
     if (!counter || name != counter->histogram_name()) \
       counter = base::LinearHistogram::FactoryGet( \
                     name, 1, boundary_value, boundary_value + 1, \
                     base::Histogram::kUmaTargetedHistogramFlag); \
     counter->Add(sample); \
   } while (0)
 
 #define CACHE_HISTOGRAM_PERCENTAGE(name, under_one_hundred) \
     CACHE_HISTOGRAM_ENUMERATION(name, under_one_hundred, 101)
 
@@ skipping 45 matching lines @@
       case net::APP_CACHE:\
         CACHE_HISTOGRAM_##type(my_name.data(), sample);\
         break;\
       default:\
         NOTREACHED();\
         break;\
     }\
   }
 
 #endif  // NET_DISK_CACHE_HISTOGRAM_MACROS_H_