gh-127266: avoid data races when updating type slots (gh-131174)

In the free-threaded build, avoid data races caused by updating type slots
or type flags after the type was initially created.  For those (typically
rare) cases, use the stop-the-world mechanism.  Remove the use of atomics
when reading or writing type flags.  The use of atomics is not sufficient to
avoid races (since flags are sometimes read without a lock and without
atomics) and are no longer required.

3 files changed, 4 insertions, 2 deletions

diff --git a/Include/internal/pycore_interp_structs.h b/Include/internal/pycore_interp_structs.h
index af6ee3ab489..f193fed1153 100644
--- a/Include/internal/pycore_interp_structs.h
+++ b/Include/internal/pycore_interp_structs.h
@@ -667,8 +667,11 @@ struct _Py_interp_cached_objects {
 
     /* object.__reduce__ */
     PyObject *objreduce;
+#ifndef Py_GIL_DISABLED
+    /* resolve_slotdups() */
     PyObject *type_slots_pname;
     pytype_slotdef *type_slots_ptrs[MAX_EQUIV];
+#endif
 
     /* TypeVar and related types */
     PyTypeObject *generic_type;
diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
index b7e162c8abc..986bcc9fd08 100644
--- a/Include/internal/pycore_object.h
+++ b/Include/internal/pycore_object.h
@@ -313,7 +313,7 @@ extern int _PyDict_CheckConsistency(PyObject *mp, int check_content);
 // Fast inlined version of PyType_HasFeature()
 static inline int
 _PyType_HasFeature(PyTypeObject *type, unsigned long feature) {
-    return ((FT_ATOMIC_LOAD_ULONG_RELAXED(type->tp_flags) & feature) != 0);
+    return ((type->tp_flags) & feature) != 0;
 }
 
 extern void _PyType_InitCache(PyInterpreterState *interp);
diff --git a/Include/internal/pycore_typeobject.h b/Include/internal/pycore_typeobject.h
index 1a4f89fd244..0ee7d555c56 100644
--- a/Include/internal/pycore_typeobject.h
+++ b/Include/internal/pycore_typeobject.h
@@ -134,7 +134,6 @@ extern int _PyType_AddMethod(PyTypeObject *, PyMethodDef *);
 extern void _PyType_SetFlagsRecursive(PyTypeObject *self, unsigned long mask,
                                       unsigned long flags);
 
-extern unsigned int _PyType_GetVersionForCurrentState(PyTypeObject *tp);
 PyAPI_FUNC(void) _PyType_SetVersion(PyTypeObject *tp, unsigned int version);
 PyTypeObject *_PyType_LookupByVersion(unsigned int version);