gh-153881: Atomically load dk_nentries in _PyObject_IsInstanceDictEmpty

Lib/test/test_free_threading/test_dict.py

@@ -336,5 +336,25 @@ def reader():
         with threading_helper.start_threads([t1, t2]):
             pass
 
+    def test_getstate_race_with_shared_keys(self):
+        box = [None]
+        enter = Barrier(2)
+        leave = Barrier(2)
+
+        def reader():
+            for _ in range(1000):
+                enter.wait()
+                box[0].__getstate__()
+                leave.wait()
+
+        def writer():
+            for i in range(1000):
+                box[0] = type(f"C{i}", (), {})()
+                enter.wait()
+                setattr(box[0], str(i), 1)
+                leave.wait()
+
+        threading_helper.run_concurrently([reader, writer])
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-07-17-22-03-39.gh-issue-153881.oDa06s.rst

@@ -0,0 +1,2 @@
+Fix potential data race when calling :meth:`~object.__getstate__`
+under the :term:`free-threaded build`.

Objects/dictobject.c

@@ -7722,7 +7722,7 @@ _PyObject_IsInstanceDictEmpty(PyObject *obj)
         PyDictValues *values = _PyObject_InlineValues(obj);
         if (FT_ATOMIC_LOAD_UINT8(values->valid)) {
             PyDictKeysObject *keys = CACHED_KEYS(tp);
-            for (Py_ssize_t i = 0; i < keys->dk_nentries; i++) {
+            for (Py_ssize_t i = 0; i < LOAD_KEYS_NENTRIES(keys); i++) {
                 if (FT_ATOMIC_LOAD_PTR_RELAXED(values->values[i]) != NULL) {
                     return 0;
                 }