Use acquire/relase for delayed persistent allocations.

base/metrics/persistent_memory_allocator.h

 // Copyright (c) 2015 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.
 
 #ifndef BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 #define BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 
 #include <stdint.h>
 
 #include <atomic>
@@ skipping 775 matching lines @@
   // offset into the segment; this allows combining allocations into a
   // single persistent segment to reduce overhead and means an "all or
   // nothing" request. Note that |size| is always the total memory size
   // and |offset| is just indicating the start of a block within it.  If
   // |make_iterable| was true, the allocation will made iterable when it
   // is created; already existing allocations are not changed.
   //
   // Once allocated, a reference to the segment will be stored at |ref|.
   // This shared location must be initialized to zero (0); it is checked
   // with every Get() request to see if the allocation has already been
-  // done.
+  // done. If reading |ref| outside of this object, be sure to do an
+  // "acquire" load. Don't write to it -- leave that to this object.
   //
   // For convenience, methods taking both Atomic32 and std::atomic<Reference>
   // are defined.
   DelayedPersistentAllocation(PersistentMemoryAllocator* allocator,
                               subtle::Atomic32* ref,
                               uint32_t type,
                               size_t size,
                               bool make_iterable);
   DelayedPersistentAllocation(PersistentMemoryAllocator* allocator,
                               subtle::Atomic32* ref,
@@ skipping 52 matching lines @@
   // share the same pointer then an allocation on one will amount to an
   // allocation for all.
   volatile std::atomic<Reference>* const reference_;
 
   // No DISALLOW_COPY_AND_ASSIGN as it's okay to copy/move these objects.
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_