From 702c5756d5a2050327603cb9e507c839f928a7a3 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Mon, 6 Nov 2017 13:44:46 +0200
Subject: Make shared_base counter atomic in C++11 and up

---
 odb/details/shared-ptr/base.cxx | 11 -----------
 odb/details/shared-ptr/base.hxx | 15 ++++++++++-----
 odb/details/shared-ptr/base.ixx | 26 +++++++++++++++++++++-----
 3 files changed, 31 insertions(+), 21 deletions(-)

diff --git a/odb/details/shared-ptr/base.cxx b/odb/details/shared-ptr/base.cxx
index e03d76e..77a571e 100644
--- a/odb/details/shared-ptr/base.cxx
+++ b/odb/details/shared-ptr/base.cxx
@@ -26,17 +26,6 @@ namespace odb
       return new not_shared (*this);
     }
 
-    bool shared_base::
-    _dec_ref_callback ()
-    {
-      bool r (--counter_ == 0);
-
-      if (r)
-        r = callback_->zero_counter (callback_->arg);
-
-      return r;
-    }
-
     namespace bits
     {
       size_t* locator_common::
diff --git a/odb/details/shared-ptr/base.hxx b/odb/details/shared-ptr/base.hxx
index c042fa6..5153aba 100644
--- a/odb/details/shared-ptr/base.hxx
+++ b/odb/details/shared-ptr/base.hxx
@@ -7,10 +7,15 @@
 
 #include <odb/pre.hxx>
 
+#include <odb/details/config.hxx> // ODB_CXX11, ODB_NOTHROW_NOEXCEPT
+
 #include <new>
 #include <cstddef>   // std::size_t
 
-#include <odb/details/config.hxx> // ODB_CXX11, ODB_NOTHROW_NOEXCEPT
+#ifdef ODB_CXX11
+#include <atomic>
+#endif
+
 #include <odb/details/export.hxx>
 #include <odb/details/shared-ptr/counter-type.hxx>
 
@@ -96,12 +101,12 @@ namespace odb
         bool (*zero_counter) (void*);
       };
 
-    private:
-      bool
-      _dec_ref_callback ();
-
     protected:
+#ifdef ODB_CXX11
+      std::atomic<std::size_t> counter_;
+#else
       std::size_t counter_;
+#endif
       refcount_callback* callback_;
     };
 
diff --git a/odb/details/shared-ptr/base.ixx b/odb/details/shared-ptr/base.ixx
index 64bc88c..16207e1 100644
--- a/odb/details/shared-ptr/base.ixx
+++ b/odb/details/shared-ptr/base.ixx
@@ -45,22 +45,38 @@ namespace odb
     inline void shared_base::
     _inc_ref ()
     {
-      counter_++;
+#ifdef ODB_CXX11
+      counter_.fetch_add (1, std::memory_order_relaxed);
+#else
+      ++counter_;
+#endif
     }
 
     inline bool shared_base::
     _dec_ref ()
     {
-      if (callback_ == 0)
-        return --counter_ == 0;
-      else
-        return _dec_ref_callback ();
+      // While there are ways to avoid acquire (which is unnecessary except
+      // when the counter drops to zero), for our use-cases we'd rather keep
+      // it simple.
+      //
+      return
+#ifdef ODB_CXX11
+        counter_.fetch_sub (1, std::memory_order_acq_rel) == 1
+#else
+        --counter_ == 0
+#endif
+        ? callback_ == 0 || callback_->zero_counter (callback_->arg)
+        : false;
     }
 
     inline std::size_t shared_base::
     _ref_count () const
     {
+#ifdef ODB_CXX11
+      return counter_.load (std::memory_order_relaxed);
+#else
       return counter_;
+#endif
     }
 
 #ifdef ODB_CXX11
-- 
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