Use lock-free lazy initialization for static histogram references

net/base/connection_type_histograms.cc

-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// 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/base/connection_type_histograms.h"
 
 #include "base/metrics/histogram.h"
 
 namespace net {
 
 // We're using a histogram as a group of counters.  We're only interested in
 // the values of the counters.  Ignore the shape, average, and standard
 // deviation of the histograms because they are meaningless.
 //
 // We use two groups of counters.  In the first group (counter1), each counter
 // is a boolean (0 or 1) that indicates whether the user has seen a connection
 // of that type during that session.  In the second group (counter2), each
 // counter is the number of connections of that type the user has seen during
 // that session.
 //
 // Each histogram has an unused bucket at the end to allow seamless future
 // expansion.
 void UpdateConnectionTypeHistograms(ConnectionType type) {
-  // TODO(wtc): Bug 74467 Move these stats up to a higher level, where the
-  // explicit static (shown below) and the implicit statics (inside the
-  // HISTOGRAM macros) will be thread safe.
-#if 0  // Don't do anything for now.
-
-  static bool had_connection_type[NUM_OF_CONNECTION_TYPES];
+  // TODO(wtc): Bug 74467 Move these stats up to a higher level.
+  static bool had_connection_type[NUM_OF_CONNECTION_TYPES];  // Default false.
 
   if (type >= 0 && type < NUM_OF_CONNECTION_TYPES) {
     if (!had_connection_type[type]) {
       had_connection_type[type] = true;
       UMA_HISTOGRAM_ENUMERATION("Net.HadConnectionType3",
           type, NUM_OF_CONNECTION_TYPES);
     }
 
     UMA_HISTOGRAM_ENUMERATION("Net.ConnectionTypeCount3",
         type, NUM_OF_CONNECTION_TYPES);
   } else {
     NOTREACHED();  // Someone's logging an invalid type!
   }
-#endif  // 0
 }
 
 }  // namespace net