gh-120198: Stop the world when setting __class__ on free-threaded build

Include/internal/pycore_dict.h

@@ -323,6 +323,8 @@ _PyInlineValuesSize(PyTypeObject *tp)
 int
 _PyDict_DetachFromObject(PyDictObject *dict, PyObject *obj);
 
+PyDictObject *_PyObject_materialize_managed_dict_lock_held(PyObject *);
+
 #ifdef __cplusplus
 }
 #endif

Include/object.h

@@ -253,11 +253,7 @@ PyAPI_FUNC(PyTypeObject*) Py_TYPE(PyObject *ob);
 #else
     static inline PyTypeObject* _Py_TYPE(PyObject *ob)
     {
-    #if defined(Py_GIL_DISABLED)
-        return (PyTypeObject *)_Py_atomic_load_ptr_relaxed(&ob->ob_type);
-    #else
         return ob->ob_type;
-    #endif
     }
     #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 < 0x030b0000
     #   define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST(ob))
@@ -288,11 +284,7 @@ static inline int Py_IS_TYPE(PyObject *ob, PyTypeObject *type) {
 
 
 static inline void Py_SET_TYPE(PyObject *ob, PyTypeObject *type) {
-#ifdef Py_GIL_DISABLED
-    _Py_atomic_store_ptr(&ob->ob_type, type);
-#else
     ob->ob_type = type;
-#endif
 }
 #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 < 0x030b0000
 #  define Py_SET_TYPE(ob, type) Py_SET_TYPE(_PyObject_CAST(ob), type)

Objects/dictobject.c

@@ -158,6 +158,10 @@ ASSERT_DICT_LOCKED(PyObject *op)
     if (!_PyInterpreterState_GET()->stoptheworld.world_stopped) {       \
         ASSERT_DICT_LOCKED(op);                                         \
     }
+#define ASSERT_WORLD_STOPPED_OR_OBJ_LOCKED(op)                         \
+    if (!_PyInterpreterState_GET()->stoptheworld.world_stopped) {      \
+        _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);                 \
+    }
 
 #define IS_DICT_SHARED(mp) _PyObject_GC_IS_SHARED(mp)
 #define SET_DICT_SHARED(mp) _PyObject_GC_SET_SHARED(mp)
@@ -227,6 +231,7 @@ static inline void split_keys_entry_added(PyDictKeysObject *keys)
 
 #define ASSERT_DICT_LOCKED(op)
 #define ASSERT_WORLD_STOPPED_OR_DICT_LOCKED(op)
+#define ASSERT_WORLD_STOPPED_OR_OBJ_LOCKED(op)
 #define LOCK_KEYS(keys)
 #define UNLOCK_KEYS(keys)
 #define ASSERT_KEYS_LOCKED(keys)
@@ -6703,10 +6708,10 @@ make_dict_from_instance_attributes(PyInterpreterState *interp,
     return res;
 }
 
-static PyDictObject *
-materialize_managed_dict_lock_held(PyObject *obj)
+PyDictObject *
+_PyObject_materialize_managed_dict_lock_held(PyObject *obj)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(obj);
+    ASSERT_WORLD_STOPPED_OR_OBJ_LOCKED(obj);
 
     PyDictValues *values = _PyObject_InlineValues(obj);
     PyInterpreterState *interp = _PyInterpreterState_GET();
@@ -6735,7 +6740,7 @@ _PyObject_MaterializeManagedDict(PyObject *obj)
         goto exit;
     }
 #endif
-    dict = materialize_managed_dict_lock_held(obj);
+    dict = _PyObject_materialize_managed_dict_lock_held(obj);
 
 #ifdef Py_GIL_DISABLED
 exit:
@@ -7170,7 +7175,7 @@ PyObject_ClearManagedDict(PyObject *obj)
 int
 _PyDict_DetachFromObject(PyDictObject *mp, PyObject *obj)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(obj);
+    ASSERT_WORLD_STOPPED_OR_OBJ_LOCKED(obj);
     assert(_PyObject_ManagedDictPointer(obj)->dict == mp);
     assert(_PyObject_InlineValuesConsistencyCheck(obj));
 

Objects/typeobject.c

@@ -6626,6 +6626,12 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
         return -1;
     }
 
+#ifdef Py_GIL_DISABLED
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    // The real Py_TYPE(self) (`oldto`) may have changed from
+    // underneath us in another thread, so we stop the world.
+    _PyEval_StopTheWorld(interp);
+#endif
     PyTypeObject *oldto = Py_TYPE(self);
 
     /* In versions of CPython prior to 3.5, the code in
@@ -6684,56 +6690,59 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
         PyErr_Format(PyExc_TypeError,
                      "__class__ assignment only supported for mutable types "
                      "or ModuleType subclasses");
-        return -1;
+        goto err;
     }
 
     if (compatible_for_assignment(oldto, newto, "__class__")) {
         /* Changing the class will change the implicit dict keys,
          * so we must materialize the dictionary first. */
         if (oldto->tp_flags & Py_TPFLAGS_INLINE_VALUES) {
-            PyDictObject *dict = _PyObject_MaterializeManagedDict(self);
+            PyDictObject *dict = _PyObject_GetManagedDict(self);
             if (dict == NULL) {
-                return -1;
+                dict = _PyObject_materialize_managed_dict_lock_held(self);
+                if (dict == NULL) {
+                    goto err;
+                }
             }
 
-            bool error = false;
-
-            Py_BEGIN_CRITICAL_SECTION2(self, dict);
-
             // If we raced after materialization and replaced the dict
             // then the materialized dict should no longer have the
             // inline values in which case detach is a nop.
+            // Note: we don't need to lock here because the world should be stopped.
+
             assert(_PyObject_GetManagedDict(self) == dict ||
                    dict->ma_values != _PyObject_InlineValues(self));
 
             if (_PyDict_DetachFromObject(dict, self) < 0) {
-                error = true;
+                goto err;
             }
 
-            Py_END_CRITICAL_SECTION2();
-            if (error) {
-                return -1;
-            }
         }
         if (newto->tp_flags & Py_TPFLAGS_HEAPTYPE) {
             Py_INCREF(newto);
         }
-        Py_BEGIN_CRITICAL_SECTION(self);
-        // The real Py_TYPE(self) (`oldto`) may have changed from
-        // underneath us in another thread, so we re-fetch it here.
-        oldto = Py_TYPE(self);
+
+#ifdef Py_GIL_DISABLED
+        _PyEval_StartTheWorld(interp);
+#endif
         Py_SET_TYPE(self, newto);
-        Py_END_CRITICAL_SECTION();
+
         if (oldto->tp_flags & Py_TPFLAGS_HEAPTYPE) {
             Py_DECREF(oldto);
         }
 
         RARE_EVENT_INC(set_class);
+
         return 0;
     }
     else {
-        return -1;
+        goto err;
     }
+err:
+#ifdef Py_GIL_DISABLED
+    _PyEval_StartTheWorld(interp);
+#endif
+    return -1;
 }
 
 static PyGetSetDef object_getsets[] = {