Specify memory ordering when reference counting with standard atomics.

Author: chriskohlhoff

include/boost/asio/detail/atomic_count.hpp

@@ -32,12 +32,31 @@ namespace detail {
 #if !defined(BOOST_ASIO_HAS_THREADS)
 typedef long atomic_count;
 inline void increment(atomic_count& a, long b) { a += b; }
+inline void ref_count_up(atomic_count& a) { ++a; }
+inline bool ref_count_down(atomic_count& a) { return --a == 0; }
 #elif defined(BOOST_ASIO_HAS_STD_ATOMIC)
 typedef std::atomic<long> atomic_count;
 inline void increment(atomic_count& a, long b) { a += b; }
+
+inline void ref_count_up(atomic_count& a)
+{
+  a.fetch_add(1, std::memory_order_relaxed);
+}
+
+inline bool ref_count_down(atomic_count& a)
+{
+  if (a.fetch_sub(1, std::memory_order_release) == 1)
+  {
+    std::atomic_thread_fence(std::memory_order_acquire);
+    return true;
+  }
+  return false;
+}
 #else // defined(BOOST_ASIO_HAS_STD_ATOMIC)
 typedef boost::detail::atomic_count atomic_count;
 inline void increment(atomic_count& a, long b) { while (b > 0) ++a, --b; }
+inline void ref_count_up(atomic_count& a) { ++a; }
+inline bool ref_count_down(atomic_count& a) { return --a == 0; }
 #endif // defined(BOOST_ASIO_HAS_STD_ATOMIC)
 
 } // namespace detail

include/boost/asio/impl/executor.hpp

@@ -149,13 +149,13 @@ class executor::impl
 
   impl_base* clone() const BOOST_ASIO_NOEXCEPT
   {
-    ++ref_count_;
+    detail::ref_count_up(ref_count_);
     return const_cast<impl_base*>(static_cast<const impl_base*>(this));
   }
 
   void destroy() BOOST_ASIO_NOEXCEPT
   {
-    if (--ref_count_ == 0)
+    if (detail::ref_count_down(ref_count_))
     {
       allocator_type alloc(allocator_);
       impl* p = this;