Stop the world when setting __class__

Author: Fidget-Spinner

Include/object.h

@@ -247,7 +247,7 @@ _Py_IsOwnedByCurrentThread(PyObject *ob)
 // bpo-39573: The Py_SET_TYPE() function must be used to set an object type.
 static inline PyTypeObject* Py_TYPE(PyObject *ob) {
 #ifdef Py_GIL_DISABLED
-    return (PyTypeObject *)_Py_atomic_load_ptr_relaxed(&ob->ob_type);
+    return (PyTypeObject *)&ob->ob_type;
 #else
     return ob->ob_type;
 #endif
@@ -278,11 +278,7 @@ 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)
@@ -7170,7 +7174,7 @@ 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

@@ -6599,6 +6599,12 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
     }
 
     PyTypeObject *oldto = Py_TYPE(self);
+#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 re-fetch it here.
+    _PyEval_StopTheWorld(interp);
+#endif
 
     /* In versions of CPython prior to 3.5, the code in
        compatible_for_assignment was not set up to correctly check for memory
@@ -6656,7 +6662,7 @@ 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__")) {
@@ -6665,47 +6671,48 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
         if (oldto->tp_flags & Py_TPFLAGS_INLINE_VALUES) {
             PyDictObject *dict = _PyObject_MaterializeManagedDict(self);
             if (dict == NULL) {
-                return -1;
+                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);
         Py_SET_TYPE(self, newto);
-        Py_END_CRITICAL_SECTION();
+
         if (oldto->tp_flags & Py_TPFLAGS_HEAPTYPE) {
             Py_DECREF(oldto);
         }
 
         RARE_EVENT_INC(set_class);
+
+#ifdef Py_GIL_DISABLED
+        _PyEval_StartTheWorld(interp);
+#endif
         return 0;
     }
     else {
-        return -1;
+        goto err;
     }
+err:
+#ifdef Py_GIL_DISABLED
+    _PyEval_StartTheWorld(interp);
+#endif
+    return -1;
 }
 
 static PyGetSetDef object_getsets[] = {