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gh-151377: Fix races updating type slots and subclasses in free-threa…
lpyu001 Jun 12, 2026
574d6a0
Create 2026-06-12-17-15-53.gh-issue-151377.bQSQUk.rst
lpyu001 Jun 12, 2026
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-17-15-53.gh-issue-151377.bQSQUk.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix races in free-threaded builds when updating type slots for newly created
classes and when removing entries from a base type's subclasses dictionary.
34 changes: 31 additions & 3 deletions Objects/typeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3943,7 +3943,7 @@ static void object_dealloc(PyObject *);
static PyObject *object_new(PyTypeObject *, PyObject *, PyObject *);
static int object_init(PyObject *, PyObject *, PyObject *);
static int update_slot(PyTypeObject *, PyObject *, slot_update_t *update);
static void fixup_slot_dispatchers(PyTypeObject *);
static int fixup_slot_dispatchers(PyTypeObject *);
static int type_new_set_names(PyTypeObject *);
static int type_new_init_subclass(PyTypeObject *, PyObject *);
static bool has_slotdef(PyObject *);
Expand Down Expand Up @@ -4954,7 +4954,9 @@ type_new_impl(type_new_ctx *ctx)
}

// Put the proper slots in place
fixup_slot_dispatchers(type);
if (fixup_slot_dispatchers(type) < 0) {
goto error;
}

if (!_PyDict_HasOnlyStringKeys(type->tp_dict)) {
if (PyErr_WarnFormat(
Expand Down Expand Up @@ -6841,7 +6843,13 @@ type_dealloc_common(PyTypeObject *type)
PyObject *bases = lookup_tp_bases(type);
if (bases != NULL) {
PyObject *exc = PyErr_GetRaisedException();
#ifdef Py_GIL_DISABLED
BEGIN_TYPE_LOCK();
#endif
remove_all_subclasses(type, bases);
#ifdef Py_GIL_DISABLED
END_TYPE_LOCK();
#endif
PyErr_SetRaisedException(exc);
}
}
Expand Down Expand Up @@ -12117,13 +12125,33 @@ update_slot(PyTypeObject *type, PyObject *name, slot_update_t *queued_updates)
/* Store the proper functions in the slot dispatches at class (type)
definition time, based upon which operations the class overrides in its
dict. */
static void
static int
fixup_slot_dispatchers(PyTypeObject *type)
{
assert(!PyErr_Occurred());
#ifdef Py_GIL_DISABLED
slot_update_t queued_updates = {0};
int res = 0;
BEGIN_TYPE_LOCK();
for (pytype_slotdef *p = slotdefs; p->name; ) {
if (update_one_slot(type, p, &p, &queued_updates) < 0) {
res = -1;
break;
}
}
if (res == 0 && queued_updates.head != NULL) {
apply_type_slot_updates(&queued_updates);
ASSERT_TYPE_LOCK_HELD();
}
END_TYPE_LOCK();
slot_update_free_chunks(&queued_updates);
return res;
#else
for (pytype_slotdef *p = slotdefs; p->name; ) {
update_one_slot(type, p, &p, NULL);
}
return 0;
#endif
}

#ifdef Py_GIL_DISABLED
Expand Down
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