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gh-111926: Avoid locking in PyType_IsSubtype (#117275) 

Read the MRO in a thread-unsafe way in `PyType_IsSubtype` to avoid locking. Fixing this is tracked in #117306. 

The motivation for this change is in support of making weakrefs thread-safe in free-threaded builds:

`WeakValueDictionary` uses a special dictionary function, `_PyDict_DelItemIf`
to remove dead weakrefs from the dictionary. `_PyDict_DelItemIf` removes a key
if a user supplied predicate evaluates to true for the value associated with
the key. Crucially for the `WeakValueDictionary` use case, the predicate
evaluation + deletion sequence is atomic, provided that the predicate doesn’t
suspend. The predicate used by `WeakValueDictionary` includes a subtype check,
which we must ensure doesn't suspend in free-threaded builds.
This commit is contained in:
mpage 2024-03-29 10:42:02 -07:00 committed by GitHub
parent 19c1dd60c5
commit 5d21d884b6
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1 changed files with 1 additions and 8 deletions
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Objects/typeobject.c
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@ -2341,14 +2341,7 @@ is_subtype_with_mro(PyObject *a_mro, PyTypeObject *a, PyTypeObject *b)
int int
PyType_IsSubtype(PyTypeObject *a, PyTypeObject *b) PyType_IsSubtype(PyTypeObject *a, PyTypeObject *b)
{ {
#ifdef Py_GIL_DISABLED return is_subtype_with_mro(a->tp_mro, a, b);
PyObject *mro = _PyType_GetMRO(a);
int res = is_subtype_with_mro(mro, a, b);
Py_XDECREF(mro);
return res;
#else
return is_subtype_with_mro(lookup_tp_mro(a), a, b);
#endif
} }
/* Routines to do a method lookup in the type without looking in the /* Routines to do a method lookup in the type without looking in the