python · kumaraditya303 · Aug 15, 2026 · Jun 12, 2026 · Aug 12, 2026 · Aug 12, 2026
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-17-15-53.gh-issue-151377.bQSQUk.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-17-15-53.gh-issue-151377.bQSQUk.rst
@@ -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.
@@ -758,17 +758,14 @@ _PyType_HasSubclasses(PyTypeObject *self)
     return 1;
 }
 
-PyObject*
-_PyType_GetSubclasses(PyTypeObject *self)
+static int
+get_subclasses_unlocked(PyTypeObject *self, PyObject *list)
 {
-    PyObject *list = PyList_New(0);
-    if (list == NULL) {
-        return NULL;
-    }
+    ASSERT_TYPE_LOCK_HELD();
 
     PyObject *subclasses = lookup_tp_subclasses(self);  // borrowed ref
     if (subclasses == NULL) {
-        return list;
+        return 0;
     }
     assert(PyDict_CheckExact(subclasses));
     // The loop cannot modify tp_subclasses, there is no need
@@ -782,12 +779,33 @@ _PyType_GetSubclasses(PyTypeObject *self)
             continue;
         }
 
-        if (PyList_Append(list, _PyObject_CAST(subclass)) < 0) {
-            Py_DECREF(list);
-            Py_DECREF(subclass);
-            return NULL;
-        }
+        int res = PyList_Append(list, _PyObject_CAST(subclass));
         Py_DECREF(subclass);
+        if (res < 0) {
+            return -1;
+        }
+    }
+    return 0;
+}
+
+PyObject*
+_PyType_GetSubclasses(PyTypeObject *self)
+{
+    PyObject *list = PyList_New(0);
+    if (list == NULL) {
+        return NULL;
+    }
+
+    // The type lock protects tp_subclasses from being mutated while we
+    // iterate over it (e.g. by add_subclass() or by remove_subclass() when a
+    // subclass is deallocated).
+    int res;
+    BEGIN_TYPE_LOCK();
+    res = get_subclasses_unlocked(self, list);
+    END_TYPE_LOCK();
+
+    if (res < 0) {
+        Py_CLEAR(list);
     }
     return list;
 }
@@ -3874,6 +3892,8 @@ 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 type_ready(PyTypeObject *, int, int);
+static int type_ready_publish(PyTypeObject *, int);
 static int type_new_set_names(PyTypeObject *);
 static int type_new_init_subclass(PyTypeObject *, PyObject *);
 static bool has_slotdef(PyObject *);
@@ -4880,13 +4900,10 @@ type_new_impl(type_new_ctx *ctx)
     }
 
     /* Initialize the rest */
-    if (PyType_Ready(type) < 0) {
+    if (type_ready_publish(type, 1) < 0) {
         goto error;
     }
 
-    // Put the proper slots in place
-    fixup_slot_dispatchers(type);
-
     if (!_PyDict_HasOnlyStringKeys(type->tp_dict)) {
         if (PyErr_WarnFormat(
                 PyExc_RuntimeWarning,
@@ -4907,10 +4924,6 @@ type_new_impl(type_new_ctx *ctx)
     }
 
     assert(_PyType_CheckConsistency(type));
-#if defined(Py_GIL_DISABLED) && defined(Py_DEBUG) && SIZEOF_VOID_P > 4
-    // After this point, other threads can potentally use this type.
-    ((PyObject*)type)->ob_flags |= _Py_TYPE_REVEALED_FLAG;
-#endif
 
     return (PyObject *)type;
 
@@ -5651,8 +5664,7 @@ type_from_slots_or_spec(
      * After this call we should generally only touch up what's
      * accessible to Python code, like __dict__.
      */
-
-    if (PyType_Ready(type) < 0) {
+    if (type_ready_publish(type, 0) < 0) {
         goto finally;
     }
 
@@ -5712,10 +5724,6 @@ type_from_slots_or_spec(
     }
 
     assert(_PyType_CheckConsistency(type));
-#if defined(Py_GIL_DISABLED) && defined(Py_DEBUG) && SIZEOF_VOID_P > 4
-    // After this point, other threads can potentally use this type.
-    ((PyObject*)type)->ob_flags |= _Py_TYPE_REVEALED_FLAG;
-#endif
 
 finally:
     if (PyErr_Occurred()) {
@@ -6692,7 +6700,9 @@ type_dealloc_common(PyTypeObject *type)
     PyObject *bases = lookup_tp_bases(type);
     if (bases != NULL) {
         PyObject *exc = PyErr_GetRaisedException();
+        BEGIN_TYPE_LOCK();
         remove_all_subclasses(type, bases);
+        END_TYPE_LOCK();
         PyErr_SetRaisedException(exc);
     }
 }
@@ -9368,7 +9378,7 @@ type_ready_post_checks(PyTypeObject *type)
 
 
 static int
-type_ready(PyTypeObject *type, int initial)
+type_ready(PyTypeObject *type, int initial, int add_subclasses)
 {
     ASSERT_TYPE_LOCK_HELD();
 
@@ -9423,8 +9433,10 @@ type_ready(PyTypeObject *type, int initial)
     if (type_ready_set_hash(type) < 0) {
         goto error;
     }
-    if (type_ready_add_subclasses(type) < 0) {
-        goto error;
+    if (add_subclasses) {
+        if (type_ready_add_subclasses(type) < 0) {
+            goto error;
+        }
     }
     if (initial) {
         if (type_ready_managed_dict(type) < 0) {
@@ -9435,11 +9447,13 @@ type_ready(PyTypeObject *type, int initial)
         }
     }
 
-    /* All done -- set the ready flag */
-    if (initial) {
-        type_add_flags(type, Py_TPFLAGS_READY);
-    } else {
-        assert(type->tp_flags & Py_TPFLAGS_READY);
+    if (add_subclasses) {
+        /* All done -- set the ready flag */
+        if (initial) {
+            type_add_flags(type, Py_TPFLAGS_READY);
+        } else {
+            assert(type->tp_flags & Py_TPFLAGS_READY);
+        }
     }
 
     stop_readying(type);
@@ -9452,6 +9466,46 @@ type_ready(PyTypeObject *type, int initial)
     return -1;
 }
 
+static int
+type_ready_publish(PyTypeObject *type, int fix_slots)
+{
+    int res;
+    BEGIN_TYPE_LOCK();
+    res = type_ready(type, 1, 0);
+    if (res == 0) {
+        assert(!(type->tp_flags & Py_TPFLAGS_READY));
+        assert(!is_readying(type));
+
+        if (fix_slots) {
+            // Put the proper slots in place and only then publish the type as
+            // a subclass of its bases.  Since the type is not reachable by
+            // other threads before it is published, the slots can be updated
+            // without stopping the world.  This step is skipped for
+            // type_from_slots_or_spec().
+            fixup_slot_dispatchers(type);
+        }
+
+        // Set the ready flag before revealing the type since type_add_flags()
+        // may only be used on types that are not yet revealed.
+        type_add_flags(type, Py_TPFLAGS_READY);
+
+#if defined(Py_GIL_DISABLED) && defined(Py_DEBUG) && SIZEOF_VOID_P > 4
+        // Mark the type as revealed while still holding the type lock.
+        // Threads can only find the type through the subclasses of its bases,
+        // which is done below with the lock held.  So, they cannot see the
+        // type before the flag is set.
+        ((PyObject*)type)->ob_flags |= _Py_TYPE_REVEALED_FLAG;
+#endif
+
+        res = type_ready_add_subclasses(type);
+        if (res == 0) {
+            assert(_PyType_CheckConsistency(type));
+        }
+    }
+    END_TYPE_LOCK();
+    return res;
+}
+
 int
 PyType_Ready(PyTypeObject *type)
 {
@@ -9471,7 +9525,7 @@ PyType_Ready(PyTypeObject *type)
     int res;
     BEGIN_TYPE_LOCK();
     if (!(type->tp_flags & Py_TPFLAGS_READY)) {
-        res = type_ready(type, 1);
+        res = type_ready(type, 1, 1);
     } else {
         res = 0;
         assert(_PyType_CheckConsistency(type));
@@ -9512,7 +9566,7 @@ init_static_type(PyInterpreterState *interp, PyTypeObject *self,
 
     int res;
     BEGIN_TYPE_LOCK();
-    res = type_ready(self, initial);
+    res = type_ready(self, initial, 1);
     END_TYPE_LOCK();
     if (res < 0) {
         _PyStaticType_ClearWeakRefs(interp, self);
@@ -11972,13 +12026,19 @@ 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. */
+   dict.  The type must not be revealed to other threads yet, so that the
+   slots can be updated directly rather than with the world stopped. */
 static void
 fixup_slot_dispatchers(PyTypeObject *type)
 {
+    ASSERT_TYPE_LOCK_HELD();
+    ASSERT_WORLD_STOPPED_OR_NEW_TYPE(type);
     assert(!PyErr_Occurred());
     for (pytype_slotdef *p = slotdefs; p->name; ) {
-        update_one_slot(type, p, &p, NULL);
+        int rv = update_one_slot(type, p, &p, NULL);
+        // always returns 0 if queued_updates == NULL
+        assert (rv == 0);
+        (void)rv;
     }
 }
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.