gh-133980: use atomic store in PyObject_GenericSetDict (#133988)

Lib/test/test_free_threading/test_dict.py

@@ -228,6 +228,22 @@ class TestDict(TestCase):
 
             self.assertEqual(count, 0)
 
+    def test_racing_object_get_set_dict(self):
+        e = Exception()
+
+        def writer():
+            for i in range(10000):
+                e.__dict__ = {1:2}
+
+        def reader():
+            for i in range(10000):
+                e.__dict__
+
+        t1 = Thread(target=writer)
+        t2 = Thread(target=reader)
+
+        with threading_helper.start_threads([t1, t2]):
+            pass
 
 if __name__ == "__main__":
     unittest.main()

Objects/object.c

@@ -1931,7 +1931,13 @@ PyObject_GenericSetDict(PyObject *obj, PyObject *value, void *context)
         return -1;
     }
     Py_BEGIN_CRITICAL_SECTION(obj);
-    Py_XSETREF(*dictptr, Py_NewRef(value));
+    PyObject *olddict = *dictptr;
+    FT_ATOMIC_STORE_PTR_RELEASE(*dictptr, Py_NewRef(value));
+#ifdef Py_GIL_DISABLED
+    _PyObject_XDecRefDelayed(olddict);
+#else
+    Py_XDECREF(olddict);
+#endif
     Py_END_CRITICAL_SECTION();
     return 0;
 }