Use atomic ops for managing g_allocator.

base/metrics/persistent_histogram_allocator.cc

 // Copyright 2016 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/persistent_histogram_allocator.h"
 
 #include <memory>
 
+#include "base/atomicops.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/important_file_writer.h"
 #include "base/files/memory_mapped_file.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
@@ skipping 18 matching lines @@
 enum : uint32_t {
   kTypeIdRangesArray = 0xBCEA225A + 1,  // SHA1(RangesArray) v1
   kTypeIdCountsArray = 0x53215530 + 1,  // SHA1(CountsArray) v1
 };
 
 // The current globally-active persistent allocator for all new histograms.
 // The object held here will obviously not be destructed at process exit
 // but that's best since PersistentMemoryAllocator objects (that underlie
 // GlobalHistogramAllocator objects) are explicitly forbidden from doing
 // anything essential at exit anyway due to the fact that they depend on data
-// managed elsewhere and which could be destructed first.
-GlobalHistogramAllocator* g_allocator = nullptr;
+// managed elsewhere and which could be destructed first. An AtomicWord is
+// used instead of std::atomic because the latter can create global ctors
+// and dtors.
+subtle::AtomicWord g_allocator = 0;
 
 // Take an array of range boundaries and create a proper BucketRanges object
 // which is returned to the caller. A return of nullptr indicates that the
 // passed boundaries are invalid.
 std::unique_ptr<BucketRanges> CreateRangesFromData(
     HistogramBase::Sample* ranges_data,
     uint32_t ranges_checksum,
     size_t count) {
   // To avoid racy destruction at shutdown, the following may be leaked.
   std::unique_ptr<BucketRanges> ranges(new BucketRanges(count));
@@ skipping 436 matching lines @@
           base::subtle::Acquire_Load(&atomic_histogram_pointer));
   if (!histogram_pointer) {
     // It's possible for multiple threads to make it here in parallel but
     // they'll always return the same result as there is a mutex in the Get.
     // The purpose of the "initialized" variable is just to ensure that
     // the same thread doesn't recurse which is also why it doesn't have
     // to be atomic.
     static bool initialized = false;
     if (!initialized) {
       initialized = true;
-      if (g_allocator) {
+      if (GlobalHistogramAllocator::Get()) {
         DVLOG(1) << "Creating the results-histogram inside persistent"
                  << " memory can cause future allocations to crash if"
                  << " that memory is ever released (for testing).";
       }
 
       histogram_pointer = LinearHistogram::FactoryGet(
           kResultHistogram, 1, CREATE_HISTOGRAM_MAX, CREATE_HISTOGRAM_MAX + 1,
           HistogramBase::kUmaTargetedHistogramFlag);
       base::subtle::Release_Store(
           &atomic_histogram_pointer,
@@ skipping 127 matching lines @@
   }
 
   return histogram;
 }
 
 HistogramBase*
 PersistentHistogramAllocator::GetOrCreateStatisticsRecorderHistogram(
     const HistogramBase* histogram) {
   // This should never be called on the global histogram allocator as objects
   // created there are already within the global statistics recorder.
-  DCHECK_NE(g_allocator, this);
+  DCHECK_NE(GlobalHistogramAllocator::Get(), this);
   DCHECK(histogram);
 
   HistogramBase* existing =
       StatisticsRecorder::FindHistogram(histogram->histogram_name());
   if (existing)
     return existing;
 
   // Adding the passed histogram to the SR would cause a problem if the
   // allocator that holds it eventually goes away. Instead, create a new
   // one from a serialized version.
@@ skipping 148 matching lines @@
       new GlobalHistogramAllocator(MakeUnique<SharedPersistentMemoryAllocator>(
           std::move(shm), 0, StringPiece(), /*readonly=*/false))));
 }
 
 // static
 void GlobalHistogramAllocator::Set(
     std::unique_ptr<GlobalHistogramAllocator> allocator) {
   // Releasing or changing an allocator is extremely dangerous because it
   // likely has histograms stored within it. If the backing memory is also
   // also released, future accesses to those histograms will seg-fault.
-  CHECK(!g_allocator);
-  g_allocator = allocator.release();
+  CHECK(!subtle::NoBarrier_Load(&g_allocator));
+  subtle::NoBarrier_Store(&g_allocator,
+                          reinterpret_cast<uintptr_t>(allocator.release()));
   size_t existing = StatisticsRecorder::GetHistogramCount();
 
   DVLOG_IF(1, existing)
       << existing << " histograms were created before persistence was enabled.";
 }
 
 // static
 GlobalHistogramAllocator* GlobalHistogramAllocator::Get() {
-  return g_allocator;
+  return reinterpret_cast<GlobalHistogramAllocator*>(
+      subtle::NoBarrier_Load(&g_allocator));
 }
 
 // static
 std::unique_ptr<GlobalHistogramAllocator>
 GlobalHistogramAllocator::ReleaseForTesting() {
-  GlobalHistogramAllocator* histogram_allocator = g_allocator;
+  GlobalHistogramAllocator* histogram_allocator = Get();
   if (!histogram_allocator)
     return nullptr;
   PersistentMemoryAllocator* memory_allocator =
       histogram_allocator->memory_allocator();
 
   // Before releasing the memory, it's necessary to have the Statistics-
   // Recorder forget about the histograms contained therein; otherwise,
   // some operations will try to access them and the released memory.
   PersistentMemoryAllocator::Iterator iter(memory_allocator);
   const PersistentHistogramData* data;
   while ((data = iter.GetNextOfObject<PersistentHistogramData>()) != nullptr) {
     StatisticsRecorder::ForgetHistogramForTesting(data->name);
 
     // If a test breaks here then a memory region containing a histogram
     // actively used by this code is being released back to the test.
     // If that memory segment were to be deleted, future calls to create
     // persistent histograms would crash. To avoid this, have the test call
     // the method GetCreateHistogramResultHistogram() *before* setting
     // the (temporary) memory allocator via SetGlobalAllocator() so that
     // histogram is instead allocated from the process heap.
     DCHECK_NE(kResultHistogram, data->name);
   }
 
-  g_allocator = nullptr;
+  subtle::NoBarrier_Store(&g_allocator, 0);
   return WrapUnique(histogram_allocator);
 };
 
 void GlobalHistogramAllocator::SetPersistentLocation(const FilePath& location) {
   persistent_location_ = location;
 }
 
 const FilePath& GlobalHistogramAllocator::GetPersistentLocation() const {
   return persistent_location_;
 }
@@ skipping 63 matching lines @@
   while (true) {
     std::unique_ptr<HistogramBase> histogram =
         import_iterator_.GetNextWithIgnore(record_to_ignore);
     if (!histogram)
       break;
     StatisticsRecorder::RegisterOrDeleteDuplicate(histogram.release());
   }
 }
 
 }  // namespace base