Use lock-free lazy initialization for static histogram references

chrome/browser/net/url_info.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 "chrome/browser/net/url_info.h"
 
 #include <math.h>
 
 #include <algorithm>
 #include <string>
@@ skipping 102 matching lines @@
   state_ = old_prequeue_state_;
   DLogResultsStats("DNS Prefetch reset to prequeue");
   static const TimeDelta kBoundary = TimeDelta::FromSeconds(2);
   if (queue_duration_ > kBoundary) {
     UMA_HISTOGRAM_MEDIUM_TIMES("DNS.QueueRecycledDeltaOver2",
                                queue_duration_ - kBoundary);
     return;
   }
   // Make a custom linear histogram for the region from 0 to boundary.
   const size_t kBucketCount = 52;
-  scoped_refptr<base::Histogram> histogram =
-      base::LinearHistogram::FactoryTimeGet(
-          "DNS.QueueRecycledUnder2", TimeDelta(), kBoundary, kBucketCount,
-          base::Histogram::kUmaTargetedHistogramFlag);
+  static base::Histogram* histogram(NULL);
+  if (!histogram)
+    histogram = base::LinearHistogram::FactoryTimeGet(
+        "DNS.QueueRecycledUnder2", TimeDelta(), kBoundary, kBucketCount,
+        base::Histogram::kUmaTargetedHistogramFlag);
   histogram->AddTime(queue_duration_);
 }
 
 void UrlInfo::SetPendingDeleteState() {
   DCHECK(ASSIGNED == state_  || ASSIGNED_BUT_MARKED == state_);
   state_ = ASSIGNED_BUT_MARKED;
 }
 
 void UrlInfo::SetFoundState() {
   DCHECK(ASSIGNED == state_);
@@ skipping 202 matching lines @@
 
     case LEARNED_REFERAL_MOTIVATED:
       return RemoveJs(referring_url_.spec());
 
     default:
       return "";
   }
 }
 
 }  // namespace chrome_browser_net