diff --git a/include/future b/include/future
index cf552a9f..f4e6fec4 100644
--- a/include/future
+++ b/include/future
@@ -470,7 +470,11 @@ public:
         {return (__state_ & __constructed) || (__exception_ != nullptr);}
 
     _LIBCPP_INLINE_VISIBILITY
-    void __set_future_attached() {__state_ |= __future_attached;}
+    void __set_future_attached()
+    {
+        lock_guard<mutex> __lk(__mut_);
+        __state_ |= __future_attached;
+    }
     _LIBCPP_INLINE_VISIBILITY
     bool __has_future_attached() const {return __state_ & __future_attached;}
 
diff --git a/src/future.cpp b/src/future.cpp
index feb37e42..7d9a5b5d 100644
--- a/src/future.cpp
+++ b/src/future.cpp
@@ -78,8 +78,8 @@ __assoc_sub_state::set_value()
         throw future_error(make_error_code(future_errc::promise_already_satisfied));
 #endif
     __state_ |= __constructed | ready;
-    __lk.unlock();
     __cv_.notify_all();
+    __lk.unlock();
 }
 
 void