gh-127266: avoid data races when updating type slots (gh-133177)

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.

Include/internal/pycore_interp_structs.h

@@ -677,8 +677,11 @@ 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;

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);

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);
 

Include/object.h

@@ -620,6 +620,12 @@ given type object has a specified feature.
 #define Py_TPFLAGS_HAVE_FINALIZE (1UL << 0)
 #define Py_TPFLAGS_HAVE_VERSION_TAG   (1UL << 18)
 
+// Flag values for ob_flags (16 bits available, if SIZEOF_VOID_P > 4).
+#define _Py_IMMORTAL_FLAGS (1 << 0)
+#define _Py_STATICALLY_ALLOCATED_FLAG (1 << 2)
+#if defined(Py_GIL_DISABLED) && defined(Py_DEBUG)
+#define _Py_TYPE_REVEALED_FLAG (1 << 3)
+#endif
 
 #define Py_CONSTANT_NONE 0
 #define Py_CONSTANT_FALSE 1
@@ -776,11 +782,7 @@ PyType_HasFeature(PyTypeObject *type, unsigned long feature)
     // PyTypeObject is opaque in the limited C API
     flags = PyType_GetFlags(type);
 #else
-#   ifdef Py_GIL_DISABLED
-        flags = _Py_atomic_load_ulong_relaxed(&type->tp_flags);
-#   else
     flags = type->tp_flags;
-#   endif
 #endif
     return ((flags & feature) != 0);
 }

Include/refcount.h

@@ -19,9 +19,6 @@ immortal. The latter should be the only instances that require
 cleanup during runtime finalization.
 */
 
-#define _Py_STATICALLY_ALLOCATED_FLAG 4
-#define _Py_IMMORTAL_FLAGS 1
-
 #if SIZEOF_VOID_P > 4
 /*
 In 64+ bit systems, any object whose 32 bit reference count is >= 2**31

Lib/test/test_descr.py

@@ -4114,6 +4114,34 @@ class ClassPropertiesAndMethods(unittest.TestCase):
         else:
             self.fail("shouldn't be able to create inheritance cycles")
 
+    def test_assign_bases_many_subclasses(self):
+        # This is intended to check that typeobject.c:queue_slot_update() can
+        # handle updating many subclasses when a slot method is re-assigned.
+        class A:
+            x = 'hello'
+            def __call__(self):
+                return 123
+            def __getitem__(self, index):
+                return None
+
+        class X:
+            x = 'bye'
+
+        class B(A):
+            pass
+
+        subclasses = []
+        for i in range(1000):
+            sc = type(f'Sub{i}', (B,), {})
+            subclasses.append(sc)
+
+        self.assertEqual(subclasses[0]()(), 123)
+        self.assertEqual(subclasses[0]().x, 'hello')
+        B.__bases__ = (X,)
+        with self.assertRaises(TypeError):
+            subclasses[0]()()
+        self.assertEqual(subclasses[0]().x, 'bye')
+
     def test_builtin_bases(self):
         # Make sure all the builtin types can have their base queried without
         # segfaulting. See issue #5787.

Misc/NEWS.d/next/Core_and_Builtins/2025-03-14-13-08-20.gh-issue-127266._tyfBp.rst

@@ -0,0 +1,6 @@
+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.

Python/ceval.c

@@ -139,6 +139,19 @@
 #endif
 
 
+static void
+check_invalid_reentrancy(void)
+{
+#if defined(Py_DEBUG) && defined(Py_GIL_DISABLED)
+    // In the free-threaded build, the interpreter must not be re-entered if
+    // the world-is-stopped.  If so, that's a bug somewhere (quite likely in
+    // the painfully complex typeobject code).
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    assert(!interp->stoptheworld.world_stopped);
+#endif
+}
+
+
 #ifdef Py_DEBUG
 static void
 dump_item(_PyStackRef item)
@@ -996,6 +1009,7 @@ PyObject* _Py_HOT_FUNCTION DONT_SLP_VECTORIZE
 _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int throwflag)
 {
     _Py_EnsureTstateNotNULL(tstate);
+    check_invalid_reentrancy();
     CALL_STAT_INC(pyeval_calls);
 
 #if USE_COMPUTED_GOTOS && !Py_TAIL_CALL_INTERP

Tools/tsan/suppressions_free_threading.txt

@@ -12,15 +12,12 @@
 
 # These warnings trigger directly in a CPython function.
 
-race_top:assign_version_tag
-race_top:_Py_slot_tp_getattr_hook
 race_top:dump_traceback
 race_top:fatal_error
 race_top:_PyFrame_GetCode
 race_top:_PyFrame_Initialize
 race_top:_PyObject_TryGetInstanceAttribute
 race_top:PyUnstable_InterpreterFrame_GetLine
-race_top:type_modified_unlocked
 race_top:write_thread_id
 
 # gh-129068: race on shared range iterators (test_free_threading.test_zip.ZipThreading.test_threading)
@@ -29,9 +26,6 @@ race_top:rangeiter_next
 # gh-129748: test.test_free_threading.test_slots.TestSlots.test_object
 race_top:mi_block_set_nextx
 
-# gh-127266: type slot updates are not thread-safe (test_opcache.test_load_attr_method_lazy_dict)
-race_top:update_one_slot
-
 # https://gist.github.com/mpage/6962e8870606cfc960e159b407a0cb40
 thread:pthread_create
 
@@ -49,7 +43,6 @@ race:list_inplace_repeat_lock_held
 race:PyObject_Realloc
 
 # gh-133467.  Some of these could be hard to trigger.
-race_top:update_one_slot
 race_top:_Py_slot_tp_getattr_hook
 race_top:slot_tp_descr_get
 race_top:type_set_name