pythongh-120198: Stop the world when setting __class__ on free-threaded build (pythonGH-120672)

Author: Fidget-Spinner

Include/internal/pycore_dict.h

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

Include/object.h

@@ -311,9 +311,9 @@ static inline Py_ssize_t Py_REFCNT(PyObject *ob) {
 #if !defined(Py_GIL_DISABLED)
     return ob->ob_refcnt;
 #else
-    uint32_t local = _Py_atomic_load_uint32_relaxed(&ob->ob_ref_local);
-    if (local == _Py_IMMORTAL_REFCNT_LOCAL) {
-        return _Py_IMMORTAL_REFCNT;
+    static inline PyTypeObject* _Py_TYPE(PyObject *ob)
+    {
+        return ob->ob_type;
     }
     Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&ob->ob_ref_shared);
     return _Py_STATIC_CAST(Py_ssize_t, local) +
@@ -421,11 +421,7 @@ static inline void Py_SET_REFCNT(PyObject *ob, Py_ssize_t refcnt) {
 
 
 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)

Lib/test/test_free_threading/test_type.py

@@ -106,7 +106,7 @@ class Bar:
         thing = Foo()
         def work():
             foo = thing
-            for _ in range(10000):
+            for _ in range(5000):
                 foo.__class__ = Bar
                 type(foo)
                 foo.__class__ = Foo

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)
@@ -6667,10 +6672,10 @@ make_dict_from_instance_attributes(PyInterpreterState *interp,
     return res;
 }
 
-static PyDictObject *
-materialize_managed_dict_lock_held(PyObject *obj)
+PyDictObject *
+_PyObject_MaterializeManagedDict_LockHeld(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();
@@ -6699,7 +6704,7 @@ _PyObject_MaterializeManagedDict(PyObject *obj)
         goto exit;
     }
 #endif
-    dict = materialize_managed_dict_lock_held(obj);
+    dict = _PyObject_MaterializeManagedDict_LockHeld(obj);
 
 #ifdef Py_GIL_DISABLED
 exit:
@@ -7132,7 +7137,7 @@ 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

@@ -6369,28 +6369,11 @@ compatible_for_assignment(PyTypeObject* oldto, PyTypeObject* newto, const char*
     return 0;
 }
 
-static int
-object_set_class(PyObject *self, PyObject *value, void *closure)
-{
-
-    if (value == NULL) {
-        PyErr_SetString(PyExc_TypeError,
-                        "can't delete __class__ attribute");
-        return -1;
-    }
-    if (!PyType_Check(value)) {
-        PyErr_Format(PyExc_TypeError,
-          "__class__ must be set to a class, not '%s' object",
-          Py_TYPE(value)->tp_name);
-        return -1;
-    }
-    PyTypeObject *newto = (PyTypeObject *)value;
 
-    if (PySys_Audit("object.__setattr__", "OsO",
-                    self, "__class__", value) < 0) {
-        return -1;
-    }
 
+static int
+object_set_class_world_stopped(PyObject *self, PyTypeObject *newto)
+{
     PyTypeObject *oldto = Py_TYPE(self);
 
     /* In versions of CPython prior to 3.5, the code in
@@ -6456,49 +6439,74 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
         /* 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_MaterializeManagedDict_LockHeld(self);
+                if (dict == NULL) {
+                    return -1;
+                }
             }
 
-            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.
-            assert(_PyObject_GetManagedDict(self) == dict ||
-                   dict->ma_values != _PyObject_InlineValues(self));
+            assert(_PyObject_GetManagedDict(self) == dict);
 
             if (_PyDict_DetachFromObject(dict, self) < 0) {
-                error = true;
-            }
-
-            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();
+
+        return 0;
+    }
+    else {
+        return -1;
+    }
+}
+
+static int
+object_set_class(PyObject *self, PyObject *value, void *closure)
+{
+
+    if (value == NULL) {
+        PyErr_SetString(PyExc_TypeError,
+                        "can't delete __class__ attribute");
+        return -1;
+    }
+    if (!PyType_Check(value)) {
+        PyErr_Format(PyExc_TypeError,
+          "__class__ must be set to a class, not '%s' object",
+          Py_TYPE(value)->tp_name);
+        return -1;
+    }
+    PyTypeObject *newto = (PyTypeObject *)value;
+
+    if (PySys_Audit("object.__setattr__", "OsO",
+                    self, "__class__", value) < 0) {
+        return -1;
+    }
+
+#ifdef Py_GIL_DISABLED
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    _PyEval_StopTheWorld(interp);
+#endif
+    PyTypeObject *oldto = Py_TYPE(self);
+    int res = object_set_class_world_stopped(self, newto);
+#ifdef Py_GIL_DISABLED
+    _PyEval_StartTheWorld(interp);
+#endif
+    if (res == 0) {
         if (oldto->tp_flags & Py_TPFLAGS_HEAPTYPE) {
             Py_DECREF(oldto);
         }
 
         RARE_EVENT_INC(set_class);
         return 0;
     }
-    else {
-        return -1;
-    }
+    return res;
 }
 
 static PyGetSetDef object_getsets[] = {