atomic_wait expects the old value, not the new one

Summary: The linked paper, the implementation and the tests use the parameter as the old value, but the parameter name is called `expected`.

Reviewed By: aary

Differential Revision: D25975108

fbshipit-source-id: bb67a95b057973e31512e9f972e1646e5a0f7d77

folly/synchronization/AtomicNotification-inl.h

@@ -51,16 +51,16 @@ extern ParkingLot<std::uint32_t> parkingLot;
 template <template <typename...> class Atom, typename... Args>
 void atomic_wait_impl(
     const Atom<std::uint32_t, Args...>* atomic,
-    std::uint32_t expected) {
-  futexWait(atomic, expected);
+    std::uint32_t old) {
+  futexWait(atomic, old);
   return;
 }
 
 template <template <typename...> class Atom, typename Integer, typename... Args>
-void atomic_wait_impl(const Atom<Integer, Args...>* atomic, Integer expected) {
+void atomic_wait_impl(const Atom<Integer, Args...>* atomic, Integer old) {
   static_assert(!std::is_same<Integer, std::uint32_t>{}, "");
   parkingLot.park(
-      atomic, -1, [&] { return atomic->load() == expected; }, [] {});
+      atomic, -1, [&] { return atomic->load() == old; }, [] {});
 }
 
 template <

folly/synchronization/AtomicNotification.h

@@ -46,11 +46,11 @@ namespace folly {
  */
 //  mimic: std::atomic_wait, p1135r0
 template <typename Integer>
-void atomic_wait(const std::atomic<Integer>* atomic, Integer expected);
+void atomic_wait(const std::atomic<Integer>* atomic, Integer old);
 template <typename Integer, typename Clock, typename Duration>
 std::cv_status atomic_wait_until(
     const std::atomic<Integer>* atomic,
-    Integer expected,
+    Integer old,
     const std::chrono::time_point<Clock, Duration>& deadline);
 
 //  mimic: std::atomic_notify_one, p1135r0