gh-118362: Fix thread safety around lookups from the type cache in the face of concurrent mutators (#118454)

Add _PyType_LookupRef and use incref before setting attribute on type
Makes setting an attribute on a class and signaling type modified atomic
Avoid adding re-entrancy exposing the type cache in an inconsistent state by decrefing after type is updated

Include/cpython/object.h

@@ -275,6 +275,7 @@ typedef struct _heaptypeobject {
 
 PyAPI_FUNC(const char *) _PyType_Name(PyTypeObject *);
 PyAPI_FUNC(PyObject *) _PyType_Lookup(PyTypeObject *, PyObject *);
+PyAPI_FUNC(PyObject *) _PyType_LookupRef(PyTypeObject *, PyObject *);
 PyAPI_FUNC(PyObject *) PyType_GetDict(PyTypeObject *);
 
 PyAPI_FUNC(int) PyObject_Print(PyObject *, FILE *, int);

Include/cpython/pyatomic.h

@@ -484,6 +484,9 @@ _Py_atomic_store_int_release(int *obj, int value);
 static inline int
 _Py_atomic_load_int_acquire(const int *obj);
 
+static inline void
+_Py_atomic_store_uint32_release(uint32_t *obj, uint32_t value);
+
 static inline void
 _Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value);
 

Include/cpython/pyatomic_gcc.h

@@ -516,6 +516,10 @@ static inline int
 _Py_atomic_load_int_acquire(const int *obj)
 { return __atomic_load_n(obj, __ATOMIC_ACQUIRE); }
 
+static inline void
+_Py_atomic_store_uint32_release(uint32_t *obj, uint32_t value)
+{ __atomic_store_n(obj, value, __ATOMIC_RELEASE); }
+
 static inline void
 _Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value)
 { __atomic_store_n(obj, value, __ATOMIC_RELEASE); }

Include/cpython/pyatomic_msc.h

@@ -989,6 +989,19 @@ _Py_atomic_load_int_acquire(const int *obj)
 #endif
 }
 
+static inline void
+_Py_atomic_store_uint32_release(uint32_t *obj, uint32_t value)
+{
+#if defined(_M_X64) || defined(_M_IX86)
+    *(uint32_t volatile *)obj = value;
+#elif defined(_M_ARM64)
+    _Py_atomic_ASSERT_ARG_TYPE(unsigned __int32);
+    __stlr32((unsigned __int32 volatile *)obj, (unsigned __int32)value);
+#else
+#  error "no implementation of _Py_atomic_store_uint32_release"
+#endif
+}
+
 static inline void
 _Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value)
 {

Include/cpython/pyatomic_std.h

@@ -911,6 +911,14 @@ _Py_atomic_load_int_acquire(const int *obj)
                                 memory_order_acquire);
 }
 
+static inline void
+_Py_atomic_store_uint32_release(uint32_t *obj, uint32_t value)
+{
+    _Py_USING_STD;
+    atomic_store_explicit((_Atomic(uint32_t)*)obj, value,
+                          memory_order_release);
+}
+
 static inline void
 _Py_atomic_store_uint64_release(uint64_t *obj, uint64_t value)
 {

Include/internal/pycore_dict.h

@@ -106,6 +106,9 @@ PyAPI_FUNC(PyObject *)_PyDict_LoadGlobal(PyDictObject *, PyDictObject *, PyObjec
 /* Consumes references to key and value */
 PyAPI_FUNC(int) _PyDict_SetItem_Take2(PyDictObject *op, PyObject *key, PyObject *value);
 extern int _PyObjectDict_SetItem(PyTypeObject *tp, PyObject **dictptr, PyObject *name, PyObject *value);
+extern int _PyDict_SetItem_LockHeld(PyDictObject *dict, PyObject *name, PyObject *value);
+extern int _PyDict_GetItemRef_Unicode_LockHeld(PyDictObject *op, PyObject *key, PyObject **result);
+extern int _PyDict_GetItemRef_KnownHash(PyDictObject *op, PyObject *key, Py_hash_t hash, PyObject **result);
 
 extern int _PyDict_Pop_KnownHash(
     PyDictObject *dict,

Include/internal/pycore_object.h

@@ -658,6 +658,7 @@ extern PyObject *_PyType_NewManagedObject(PyTypeObject *type);
 extern PyTypeObject* _PyType_CalculateMetaclass(PyTypeObject *, PyObject *);
 extern PyObject* _PyType_GetDocFromInternalDoc(const char *, const char *);
 extern PyObject* _PyType_GetTextSignatureFromInternalDoc(const char *, const char *, int);
+extern int _PyObject_SetAttributeErrorContext(PyObject *v, PyObject* name);
 
 void _PyObject_InitInlineValues(PyObject *obj, PyTypeObject *tp);
 extern int _PyObject_StoreInstanceAttribute(PyObject *obj,

Lib/test/test_class.py

@@ -882,6 +882,25 @@ class TestInlineValues(unittest.TestCase):
         f.a = 3
         self.assertEqual(f.a, 3)
 
+    def test_store_attr_type_cache(self):
+        """Verifies that the type cache doesn't provide a value which  is
+        inconsistent from the dict."""
+        class X:
+            def __del__(inner_self):
+                v = C.a
+                self.assertEqual(v, C.__dict__['a'])
+
+        class C:
+            a = X()
+
+        # prime the cache
+        C.a
+        C.a
+
+        # destructor shouldn't be able to see inconsisent state
+        C.a = X()
+        C.a = X()
+
 
 if __name__ == '__main__':
     unittest.main()

Lib/test/test_free_threading/test_type.py

@@ -0,0 +1,112 @@
+import unittest
+
+from threading import Thread
+from multiprocessing.dummy import Pool
+from unittest import TestCase
+
+from test.support import is_wasi
+
+
+NTHREADS = 6
+BOTTOM = 0
+TOP = 1000
+ITERS = 100
+
+class A:
+    attr = 1
+
+@unittest.skipIf(is_wasi, "WASI has no threads.")
+class TestType(TestCase):
+    def test_attr_cache(self):
+        def read(id0):
+            for _ in range(ITERS):
+                for _ in range(BOTTOM, TOP):
+                    A.attr
+
+        def write(id0):
+            for _ in range(ITERS):
+                for _ in range(BOTTOM, TOP):
+                    # Make _PyType_Lookup cache hot first
+                    A.attr
+                    A.attr
+                    x = A.attr
+                    x += 1
+                    A.attr = x
+
+
+        with Pool(NTHREADS) as pool:
+            pool.apply_async(read, (1,))
+            pool.apply_async(write, (1,))
+            pool.close()
+            pool.join()
+
+    def test_attr_cache_consistency(self):
+        class C:
+            x = 0
+
+        DONE = False
+        def writer_func():
+            for i in range(3000):
+                C.x
+                C.x
+                C.x += 1
+            nonlocal DONE
+            DONE = True
+
+        def reader_func():
+            while True:
+                # We should always see a greater value read from the type than the
+                # dictionary
+                a = C.__dict__['x']
+                b = C.x
+                self.assertGreaterEqual(b, a)
+
+                if DONE:
+                    break
+
+        self.run_one(writer_func, reader_func)
+
+    def test_attr_cache_consistency_subclass(self):
+        class C:
+            x = 0
+
+        class D(C):
+            pass
+
+        DONE = False
+        def writer_func():
+            for i in range(3000):
+                D.x
+                D.x
+                C.x += 1
+            nonlocal DONE
+            DONE = True
+
+        def reader_func():
+            while True:
+                # We should always see a greater value read from the type than the
+                # dictionary
+                a = C.__dict__['x']
+                b = D.x
+                self.assertGreaterEqual(b, a)
+
+                if DONE:
+                    break
+
+        self.run_one(writer_func, reader_func)
+
+    def run_one(self, writer_func, reader_func):
+        writer = Thread(target=writer_func)
+        readers = []
+        for x in range(30):
+            reader = Thread(target=reader_func)
+            readers.append(reader)
+            reader.start()
+
+        writer.start()
+        writer.join()
+        for reader in readers:
+            reader.join()
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Core and Builtins/2024-05-01-14-20-28.gh-issue-118492.VUsSfn.rst

@@ -0,0 +1,2 @@
+Fix an issue where the type cache can expose a previously accessed attribute
+when a finalizer is run.

Modules/_collectionsmodule.c

@@ -2511,9 +2511,9 @@ _collections__count_elements_impl(PyObject *module, PyObject *mapping,
     /* Only take the fast path when get() and __setitem__()
      * have not been overridden.
      */
-    mapping_get = _PyType_Lookup(Py_TYPE(mapping), &_Py_ID(get));
+    mapping_get = _PyType_LookupRef(Py_TYPE(mapping), &_Py_ID(get));
     dict_get = _PyType_Lookup(&PyDict_Type, &_Py_ID(get));
-    mapping_setitem = _PyType_Lookup(Py_TYPE(mapping), &_Py_ID(__setitem__));
+    mapping_setitem = _PyType_LookupRef(Py_TYPE(mapping), &_Py_ID(__setitem__));
     dict_setitem = _PyType_Lookup(&PyDict_Type, &_Py_ID(__setitem__));
 
     if (mapping_get != NULL && mapping_get == dict_get &&
@@ -2587,6 +2587,8 @@ _collections__count_elements_impl(PyObject *module, PyObject *mapping,
     }
 
 done:
+    Py_XDECREF(mapping_get);
+    Py_XDECREF(mapping_setitem);
     Py_DECREF(it);
     Py_XDECREF(key);
     Py_XDECREF(newval);

Modules/_ctypes/_ctypes.c

@@ -1096,7 +1096,7 @@ static int
 UnionType_setattro(PyObject *self, PyObject *key, PyObject *value)
 {
     /* XXX Should we disallow deleting _fields_? */
-    if (-1 == PyObject_GenericSetAttr(self, key, value))
+    if (-1 == PyType_Type.tp_setattro(self, key, value))
         return -1;
 
     if (PyUnicode_Check(key) &&

Modules/_lsprof.c

@@ -177,8 +177,7 @@ normalizeUserObj(PyObject *obj)
         PyObject *modname = fn->m_module;
 
         if (name != NULL) {
-            PyObject *mo = _PyType_Lookup(Py_TYPE(self), name);
+            PyObject *mo = _PyType_LookupRef(Py_TYPE(self), name);
-            Py_XINCREF(mo);
             Py_DECREF(name);
             if (mo != NULL) {
                 PyObject *res = PyObject_Repr(mo);

Modules/_testcapi/gc.c

@@ -99,10 +99,11 @@ slot_tp_del(PyObject *self)
         return;
     }
     /* Execute __del__ method, if any. */
-    del = _PyType_Lookup(Py_TYPE(self), tp_del);
+    del = _PyType_LookupRef(Py_TYPE(self), tp_del);
     Py_DECREF(tp_del);
     if (del != NULL) {
         res = PyObject_CallOneArg(del, self);
+        Py_DECREF(del);
         if (res == NULL)
             PyErr_WriteUnraisable(del);
         else

Objects/classobject.c

@@ -188,15 +188,18 @@ method_getattro(PyObject *obj, PyObject *name)
             if (PyType_Ready(tp) < 0)
                 return NULL;
         }
-        descr = _PyType_Lookup(tp, name);
+        descr = _PyType_LookupRef(tp, name);
     }
 
     if (descr != NULL) {
         descrgetfunc f = TP_DESCR_GET(Py_TYPE(descr));
-        if (f != NULL)
+        if (f != NULL) {
-            return f(descr, obj, (PyObject *)Py_TYPE(obj));
+            PyObject *res = f(descr, obj, (PyObject *)Py_TYPE(obj));
+            Py_DECREF(descr);
+            return res;
+        }
         else {
-            return Py_NewRef(descr);
+            return descr;
         }
     }
 
@@ -410,14 +413,17 @@ instancemethod_getattro(PyObject *self, PyObject *name)
         if (PyType_Ready(tp) < 0)
             return NULL;
     }
-    descr = _PyType_Lookup(tp, name);
+    descr = _PyType_LookupRef(tp, name);
 
     if (descr != NULL) {
         descrgetfunc f = TP_DESCR_GET(Py_TYPE(descr));
-        if (f != NULL)
+        if (f != NULL) {
-            return f(descr, self, (PyObject *)Py_TYPE(self));
+            PyObject *res = f(descr, self, (PyObject *)Py_TYPE(self));
+            Py_DECREF(descr);
+            return res;
+        }
         else {
-            return Py_NewRef(descr);
+            return descr;
         }
     }
 

Objects/dictobject.c

@@ -2268,15 +2268,13 @@ _PyDict_GetItem_KnownHash(PyObject *op, PyObject *key, Py_hash_t hash)
  * exception occurred.
 */
 int
-_PyDict_GetItemRef_KnownHash(PyObject *op, PyObject *key, Py_hash_t hash, PyObject **result)
+_PyDict_GetItemRef_KnownHash(PyDictObject *op, PyObject *key, Py_hash_t hash, PyObject **result)
 {
-    PyDictObject*mp = (PyDictObject *)op;
-
     PyObject *value;
 #ifdef Py_GIL_DISABLED
-    Py_ssize_t ix = _Py_dict_lookup_threadsafe(mp, key, hash, &value);
+    Py_ssize_t ix = _Py_dict_lookup_threadsafe(op, key, hash, &value);
 #else
-    Py_ssize_t ix = _Py_dict_lookup(mp, key, hash, &value);
+    Py_ssize_t ix = _Py_dict_lookup(op, key, hash, &value);
 #endif
     assert(ix >= 0 || value == NULL);
     if (ix == DKIX_ERROR) {
@@ -2314,9 +2312,38 @@ PyDict_GetItemRef(PyObject *op, PyObject *key, PyObject **result)
         }
     }
 
-    return _PyDict_GetItemRef_KnownHash(op, key, hash, result);
+    return _PyDict_GetItemRef_KnownHash((PyDictObject *)op, key, hash, result);
 }
 
+int
+_PyDict_GetItemRef_Unicode_LockHeld(PyDictObject *op, PyObject *key, PyObject **result)
+{
+    ASSERT_DICT_LOCKED(op);
+    assert(PyUnicode_CheckExact(key));
+
+    Py_hash_t hash;
+    if ((hash = unicode_get_hash(key)) == -1) {
+        hash = PyObject_Hash(key);
+        if (hash == -1) {
+            *result = NULL;
+            return -1;
+        }
+    }
+
+    PyObject *value;
+    Py_ssize_t ix = _Py_dict_lookup(op, key, hash, &value);
+    assert(ix >= 0 || value == NULL);
+    if (ix == DKIX_ERROR) {
+        *result = NULL;
+        return -1;
+    }
+    if (value == NULL) {
+        *result = NULL;
+        return 0;  // missing key
+    }
+    *result = Py_NewRef(value);
+    return 1;  // key is present
+}
 
 /* Variant of PyDict_GetItem() that doesn't suppress exceptions.
    This returns NULL *with* an exception set if an exception occurred.
@@ -6704,8 +6731,8 @@ exit:
     return dict;
 }
 
-static int
+int
-set_or_del_lock_held(PyDictObject *dict, PyObject *name, PyObject *value)
+_PyDict_SetItem_LockHeld(PyDictObject *dict, PyObject *name, PyObject *value)
 {
     if (value == NULL) {
         Py_hash_t hash;
@@ -6767,7 +6794,7 @@ store_instance_attr_lock_held(PyObject *obj, PyDictValues *values,
             // so that no one else will see it.
             dict = make_dict_from_instance_attributes(PyInterpreterState_Get(), keys, values);
             if (dict == NULL ||
-                set_or_del_lock_held(dict, name, value) < 0) {
+                _PyDict_SetItem_LockHeld(dict, name, value) < 0) {
                 Py_XDECREF(dict);
                 return -1;
             }
@@ -6779,7 +6806,7 @@ store_instance_attr_lock_held(PyObject *obj, PyDictValues *values,
 
         _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(dict);
 
-        res = set_or_del_lock_held (dict, name, value);
+        res = _PyDict_SetItem_LockHeld(dict, name, value);
         return res;
     }
 
@@ -6822,7 +6849,7 @@ store_instance_attr_dict(PyObject *obj, PyDictObject *dict, PyObject *name, PyOb
         res = store_instance_attr_lock_held(obj, values, name, value);
     }
     else {
-        res = set_or_del_lock_held(dict, name, value);
+        res = _PyDict_SetItem_LockHeld(dict, name, value);
     }
     Py_END_CRITICAL_SECTION();
     return res;
@@ -7253,6 +7280,7 @@ _PyObjectDict_SetItem(PyTypeObject *tp, PyObject **dictptr,
             res = PyDict_SetItem(dict, key, value);
         }
     }
+
     ASSERT_CONSISTENT(dict);
     return res;
 }

Objects/object.c

@@ -1132,8 +1132,8 @@ _PyObject_SetAttrId(PyObject *v, _Py_Identifier *name, PyObject *w)
     return result;
 }
 
-static inline int
+int
-set_attribute_error_context(PyObject* v, PyObject* name)
+_PyObject_SetAttributeErrorContext(PyObject* v, PyObject* name)
 {
     assert(PyErr_Occurred());
     if (!PyErr_ExceptionMatches(PyExc_AttributeError)){
@@ -1188,7 +1188,7 @@ PyObject_GetAttr(PyObject *v, PyObject *name)
     }
 
     if (result == NULL) {
-        set_attribute_error_context(v, name);
+        _PyObject_SetAttributeErrorContext(v, name);
     }
     return result;
 }
@@ -1466,13 +1466,13 @@ _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method)
         return 0;
     }
 
-    PyObject *descr = _PyType_Lookup(tp, name);
+    PyObject *descr = _PyType_LookupRef(tp, name);
     descrgetfunc f = NULL;
     if (descr != NULL) {
-        Py_INCREF(descr);
         if (_PyType_HasFeature(Py_TYPE(descr), Py_TPFLAGS_METHOD_DESCRIPTOR)) {
             meth_found = 1;
-        } else {
+        }
+        else {
             f = Py_TYPE(descr)->tp_descr_get;
             if (f != NULL && PyDescr_IsData(descr)) {
                 *method = f(descr, obj, (PyObject *)Py_TYPE(obj));
@@ -1535,7 +1535,7 @@ _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method)
                  "'%.100s' object has no attribute '%U'",
                  tp->tp_name, name);
 
-    set_attribute_error_context(obj, name);
+    _PyObject_SetAttributeErrorContext(obj, name);
     return 0;
 }
 
@@ -1569,11 +1569,10 @@ _PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name,
             goto done;
     }
 
-    descr = _PyType_Lookup(tp, name);
+    descr = _PyType_LookupRef(tp, name);
 
     f = NULL;
     if (descr != NULL) {
-        Py_INCREF(descr);
         f = Py_TYPE(descr)->tp_descr_get;
         if (f != NULL && PyDescr_IsData(descr)) {
             res = f(descr, obj, (PyObject *)Py_TYPE(obj));
@@ -1647,7 +1646,7 @@ _PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name,
                      "'%.100s' object has no attribute '%U'",
                      tp->tp_name, name);
 
-        set_attribute_error_context(obj, name);
+        _PyObject_SetAttributeErrorContext(obj, name);
     }
   done:
     Py_XDECREF(descr);
@@ -1670,6 +1669,7 @@ _PyObject_GenericSetAttrWithDict(PyObject *obj, PyObject *name,
     descrsetfunc f;
     int res = -1;
 
+    assert(!PyType_IsSubtype(tp, &PyType_Type));
     if (!PyUnicode_Check(name)){
         PyErr_Format(PyExc_TypeError,
                      "attribute name must be string, not '%.200s'",
@@ -1683,10 +1683,9 @@ _PyObject_GenericSetAttrWithDict(PyObject *obj, PyObject *name,
 
     Py_INCREF(name);
     Py_INCREF(tp);
-    descr = _PyType_Lookup(tp, name);
+    descr = _PyType_LookupRef(tp, name);
 
     if (descr != NULL) {
-        Py_INCREF(descr);
         f = Py_TYPE(descr)->tp_descr_set;
         if (f != NULL) {
             res = f(descr, obj, value);
@@ -1722,7 +1721,7 @@ _PyObject_GenericSetAttrWithDict(PyObject *obj, PyObject *name,
                                 "'%.100s' object has no attribute '%U'",
                                 tp->tp_name, name);
                 }
-                set_attribute_error_context(obj, name);
+                _PyObject_SetAttributeErrorContext(obj, name);
             }
             else {
                 PyErr_Format(PyExc_AttributeError,
@@ -1745,17 +1744,10 @@ _PyObject_GenericSetAttrWithDict(PyObject *obj, PyObject *name,
     }
   error_check:
     if (res < 0 && PyErr_ExceptionMatches(PyExc_KeyError)) {
-        if (PyType_IsSubtype(tp, &PyType_Type)) {
+        PyErr_Format(PyExc_AttributeError,
-            PyErr_Format(PyExc_AttributeError,
+                        "'%.100s' object has no attribute '%U'",
-                         "type object '%.50s' has no attribute '%U'",
+                        tp->tp_name, name);
-                         ((PyTypeObject*)obj)->tp_name, name);
+        _PyObject_SetAttributeErrorContext(obj, name);
-        }
-        else {
-            PyErr_Format(PyExc_AttributeError,
-                         "'%.100s' object has no attribute '%U'",
-                         tp->tp_name, name);
-        }
-        set_attribute_error_context(obj, name);
     }
   done:
     Py_XDECREF(descr);

Objects/typeobject.c

@@ -71,6 +71,16 @@ class object "PyObject *" "&PyBaseObject_Type"
         _PyCriticalSection_End(&_cs);                                   \
     }
 
+#define BEGIN_TYPE_DICT_LOCK(d)                                         \
+    {                                                                   \
+        _PyCriticalSection2 _cs;                                        \
+        _PyCriticalSection2_Begin(&_cs, TYPE_LOCK,                      \
+                                  &_PyObject_CAST(d)->ob_mutex);        \
+
+#define END_TYPE_DICT_LOCK()                                            \
+        _PyCriticalSection2_End(&_cs);                                  \
+    }
+
 #define ASSERT_TYPE_LOCK_HELD() \
     _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(TYPE_LOCK)
 
@@ -78,6 +88,8 @@ class object "PyObject *" "&PyBaseObject_Type"
 
 #define BEGIN_TYPE_LOCK()
 #define END_TYPE_LOCK()
+#define BEGIN_TYPE_DICT_LOCK(d)
+#define END_TYPE_DICT_LOCK()
 #define ASSERT_TYPE_LOCK_HELD()
 
 #endif
@@ -728,7 +740,7 @@ _PyType_InitCache(PyInterpreterState *interp)
         assert(entry->name == NULL);
 
         entry->version = 0;
-        // Set to None so _PyType_Lookup() can use Py_SETREF(),
+        // Set to None so _PyType_LookupRef() can use Py_SETREF(),
         // rather than using slower Py_XSETREF().
         entry->name = Py_None;
         entry->value = NULL;
@@ -740,7 +752,7 @@ static unsigned int
 _PyType_ClearCache(PyInterpreterState *interp)
 {
     struct type_cache *cache = &interp->types.type_cache;
-    // Set to None, rather than NULL, so _PyType_Lookup() can
+    // Set to None, rather than NULL, so _PyType_LookupRef() can
     // use Py_SETREF() rather than using slower Py_XSETREF().
     type_cache_clear(cache, Py_None);
 
@@ -849,6 +861,44 @@ PyType_Unwatch(int watcher_id, PyObject* obj)
     return 0;
 }
 
+#ifdef Py_GIL_DISABLED
+
+static void
+type_modification_starting_unlocked(PyTypeObject *type)
+{
+    ASSERT_TYPE_LOCK_HELD();
+
+    /* Clear version tags on all types, but leave the valid
+       version tag intact.  This prepares for a modification so that
+       any concurrent readers of the type cache will not see invalid
+       values.
+     */
+    if (!_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG)) {
+        return;
+    }
+
+    PyObject *subclasses = lookup_tp_subclasses(type);
+    if (subclasses != NULL) {
+        assert(PyDict_CheckExact(subclasses));
+
+        Py_ssize_t i = 0;
+        PyObject *ref;
+        while (PyDict_Next(subclasses, &i, NULL, &ref)) {
+            PyTypeObject *subclass = type_from_ref(ref);
+            if (subclass == NULL) {
+                continue;
+            }
+            type_modification_starting_unlocked(subclass);
+            Py_DECREF(subclass);
+        }
+    }
+
+    /* 0 is not a valid version tag */
+    _Py_atomic_store_uint32_release(&type->tp_version_tag, 0);
+}
+
+#endif
+
 static void
 type_modified_unlocked(PyTypeObject *type)
 {
@@ -882,7 +932,7 @@ type_modified_unlocked(PyTypeObject *type)
             if (subclass == NULL) {
                 continue;
             }
-            PyType_Modified(subclass);
+            type_modified_unlocked(subclass);
             Py_DECREF(subclass);
         }
     }
@@ -2352,7 +2402,7 @@ PyType_IsSubtype(PyTypeObject *a, PyTypeObject *b)
    Variants:
 
    - _PyObject_LookupSpecial() returns NULL without raising an exception
-     when the _PyType_Lookup() call fails;
+     when the _PyType_LookupRef() call fails;
 
    - lookup_maybe_method() and lookup_method() are internal routines similar
      to _PyObject_LookupSpecial(), but can return unbound PyFunction
@@ -2365,13 +2415,12 @@ _PyObject_LookupSpecial(PyObject *self, PyObject *attr)
 {
     PyObject *res;
 
-    res = _PyType_Lookup(Py_TYPE(self), attr);
+    res = _PyType_LookupRef(Py_TYPE(self), attr);
     if (res != NULL) {
         descrgetfunc f;
-        if ((f = Py_TYPE(res)->tp_descr_get) == NULL)
+        if ((f = Py_TYPE(res)->tp_descr_get) != NULL) {
-            Py_INCREF(res);
+            Py_SETREF(res, f(res, self, (PyObject *)(Py_TYPE(self))));
-        else
+        }
-            res = f(res, self, (PyObject *)(Py_TYPE(self)));
     }
     return res;
 }
@@ -2388,7 +2437,7 @@ _PyObject_LookupSpecialId(PyObject *self, _Py_Identifier *attrid)
 static PyObject *
 lookup_maybe_method(PyObject *self, PyObject *attr, int *unbound)
 {
-    PyObject *res = _PyType_Lookup(Py_TYPE(self), attr);
+    PyObject *res = _PyType_LookupRef(Py_TYPE(self), attr);
     if (res == NULL) {
         return NULL;
     }
@@ -2396,16 +2445,12 @@ lookup_maybe_method(PyObject *self, PyObject *attr, int *unbound)
     if (_PyType_HasFeature(Py_TYPE(res), Py_TPFLAGS_METHOD_DESCRIPTOR)) {
         /* Avoid temporary PyMethodObject */
         *unbound = 1;
-        Py_INCREF(res);
     }
     else {
         *unbound = 0;
         descrgetfunc f = Py_TYPE(res)->tp_descr_get;
-        if (f == NULL) {
+        if (f != NULL) {
-            Py_INCREF(res);
+            Py_SETREF(res, f(res, self, (PyObject *)(Py_TYPE(self))));
-        }
-        else {
-            res = f(res, self, (PyObject *)(Py_TYPE(self)));
         }
     }
     return res;
@@ -4981,14 +5026,13 @@ find_name_in_mro(PyTypeObject *type, PyObject *name, int *error)
         PyObject *base = PyTuple_GET_ITEM(mro, i);
         PyObject *dict = lookup_tp_dict(_PyType_CAST(base));
         assert(dict && PyDict_Check(dict));
-        res = _PyDict_GetItem_KnownHash(dict, name, hash);
+        if (_PyDict_GetItemRef_KnownHash((PyDictObject *)dict, name, hash, &res) < 0) {
-        if (res != NULL) {
-            break;
-        }
-        if (PyErr_Occurred()) {
             *error = -1;
             goto done;
         }
+        if (res != NULL) {
+            break;
+        }
     }
     *error = 0;
 done:
@@ -5030,8 +5074,6 @@ update_cache(struct type_cache_entry *entry, PyObject *name, unsigned int versio
 
 #if Py_GIL_DISABLED
 
-#define TYPE_CACHE_IS_UPDATING(sequence) (sequence & 0x01)
-
 static void
 update_cache_gil_disabled(struct type_cache_entry *entry, PyObject *name,
                           unsigned int version_tag, PyObject *value)
@@ -5075,7 +5117,7 @@ _PyTypes_AfterFork(void)
 /* Internal API to look for a name through the MRO.
    This returns a borrowed reference, and doesn't set an exception! */
 PyObject *
-_PyType_Lookup(PyTypeObject *type, PyObject *name)
+_PyType_LookupRef(PyTypeObject *type, PyObject *name)
 {
     PyObject *res;
     int error;
@@ -5089,19 +5131,25 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
     while (1) {
         int sequence = _PySeqLock_BeginRead(&entry->sequence);
         uint32_t entry_version = _Py_atomic_load_uint32_relaxed(&entry->version);
-        uint32_t type_version = _Py_atomic_load_uint32_relaxed(&type->tp_version_tag);
+        uint32_t type_version = _Py_atomic_load_uint32_acquire(&type->tp_version_tag);
         if (entry_version == type_version &&
             _Py_atomic_load_ptr_relaxed(&entry->name) == name) {
-            assert(_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG));
             OBJECT_STAT_INC_COND(type_cache_hits, !is_dunder_name(name));
             OBJECT_STAT_INC_COND(type_cache_dunder_hits, is_dunder_name(name));
             PyObject *value = _Py_atomic_load_ptr_relaxed(&entry->value);
-
             // If the sequence is still valid then we're done
-            if (_PySeqLock_EndRead(&entry->sequence, sequence)) {
+            if (value == NULL || _Py_TryIncref(value)) {
-                return value;
+                if (_PySeqLock_EndRead(&entry->sequence, sequence)) {
+                    return value;
+                }
+                Py_XDECREF(value);
             }
-        } else {
+            else {
+                // If we can't incref the object we need to fallback to locking
+                break;
+            }
+        }
+        else {
             // cache miss
             break;
         }
@@ -5112,6 +5160,7 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
         assert(_PyType_HasFeature(type, Py_TPFLAGS_VALID_VERSION_TAG));
         OBJECT_STAT_INC_COND(type_cache_hits, !is_dunder_name(name));
         OBJECT_STAT_INC_COND(type_cache_dunder_hits, is_dunder_name(name));
+        Py_XINCREF(entry->value);
         return entry->value;
     }
 #endif
@@ -5161,6 +5210,14 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
     return res;
 }
 
+PyObject *
+_PyType_Lookup(PyTypeObject *type, PyObject *name)
+{
+    PyObject *res = _PyType_LookupRef(type, name);
+    Py_XDECREF(res);
+    return res;
+}
+
 PyObject *
 _PyType_LookupId(PyTypeObject *type, _Py_Identifier *name)
 {
@@ -5218,7 +5275,7 @@ _PyType_SetFlagsRecursive(PyTypeObject *self, unsigned long mask, unsigned long
 }
 
 /* This is similar to PyObject_GenericGetAttr(),
-   but uses _PyType_Lookup() instead of just looking in type->tp_dict.
+   but uses _PyType_LookupRef() instead of just looking in type->tp_dict.
 
    The argument suppress_missing_attribute is used to provide a
    fast path for hasattr. The possible values are:
@@ -5254,10 +5311,9 @@ _Py_type_getattro_impl(PyTypeObject *type, PyObject *name, int * suppress_missin
     meta_get = NULL;
 
     /* Look for the attribute in the metatype */
-    meta_attribute = _PyType_Lookup(metatype, name);
+    meta_attribute = _PyType_LookupRef(metatype, name);
 
     if (meta_attribute != NULL) {
-        Py_INCREF(meta_attribute);
         meta_get = Py_TYPE(meta_attribute)->tp_descr_get;
 
         if (meta_get != NULL && PyDescr_IsData(meta_attribute)) {
@@ -5274,10 +5330,9 @@ _Py_type_getattro_impl(PyTypeObject *type, PyObject *name, int * suppress_missin
 
     /* No data descriptor found on metatype. Look in tp_dict of this
      * type and its bases */
-    attribute = _PyType_Lookup(type, name);
+    attribute = _PyType_LookupRef(type, name);
     if (attribute != NULL) {
         /* Implement descriptor functionality, if any */
-        Py_INCREF(attribute);
         descrgetfunc local_get = Py_TYPE(attribute)->tp_descr_get;
 
         Py_XDECREF(meta_attribute);
@@ -5322,7 +5377,7 @@ _Py_type_getattro_impl(PyTypeObject *type, PyObject *name, int * suppress_missin
 }
 
 /* This is similar to PyObject_GenericGetAttr(),
-   but uses _PyType_Lookup() instead of just looking in type->tp_dict. */
+   but uses _PyType_LookupRef() instead of just looking in type->tp_dict. */
 PyObject *
 _Py_type_getattro(PyObject *type, PyObject *name)
 {
@@ -5341,49 +5396,110 @@ type_setattro(PyObject *self, PyObject *name, PyObject *value)
             name, type->tp_name);
         return -1;
     }
-    if (PyUnicode_Check(name)) {
+    if (!PyUnicode_Check(name)) {
-        if (PyUnicode_CheckExact(name)) {
+        PyErr_Format(PyExc_TypeError,
-            Py_INCREF(name);
+                     "attribute name must be string, not '%.200s'",
-        }
+                     Py_TYPE(name)->tp_name);
-        else {
+        return -1;
-            name = _PyUnicode_Copy(name);
-            if (name == NULL)
-                return -1;
-        }
-        /* bpo-40521: Interned strings are shared by all subinterpreters */
-        if (!PyUnicode_CHECK_INTERNED(name)) {
-            PyUnicode_InternInPlace(&name);
-            if (!PyUnicode_CHECK_INTERNED(name)) {
-                PyErr_SetString(PyExc_MemoryError,
-                                "Out of memory interning an attribute name");
-                Py_DECREF(name);
-                return -1;
-            }
-        }
     }
-    else {
+
-        /* Will fail in _PyObject_GenericSetAttrWithDict. */
+    if (PyUnicode_CheckExact(name)) {
         Py_INCREF(name);
     }
+    else {
+        name = _PyUnicode_Copy(name);
+        if (name == NULL)
+            return -1;
+    }
+    /* bpo-40521: Interned strings are shared by all subinterpreters */
+    if (!PyUnicode_CHECK_INTERNED(name)) {
+        PyUnicode_InternInPlace(&name);
+        if (!PyUnicode_CHECK_INTERNED(name)) {
+            PyErr_SetString(PyExc_MemoryError,
+                            "Out of memory interning an attribute name");
+            Py_DECREF(name);
+            return -1;
+        }
+    }
+
+    PyTypeObject *metatype = Py_TYPE(type);
+    assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_INLINE_VALUES));
+    assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_MANAGED_DICT));
+
+    PyObject *old_value;
+    PyObject *descr = _PyType_LookupRef(metatype, name);
+    if (descr != NULL) {
+        descrsetfunc f = Py_TYPE(descr)->tp_descr_set;
+        if (f != NULL) {
+            old_value = NULL;
+            res = f(descr, (PyObject *)type, value);
+            goto done;
+        }
+    }
+
+    PyObject *dict = type->tp_dict;
+    if (dict == NULL) {
+        // We don't just do PyType_Ready because we could already be readying
+        BEGIN_TYPE_LOCK();
+        dict = type->tp_dict;
+        if (dict == NULL) {
+            dict = type->tp_dict = PyDict_New();
+        }
+        END_TYPE_LOCK();
+        if (dict == NULL) {
+            return -1;
+        }
+    }
+
+    // We don't want any re-entrancy between when we update the dict
+    // and call type_modified_unlocked, including running the destructor
+    // of the current value as it can observe the cache in an inconsistent
+    // state.  Because we have an exact unicode and our dict has exact
+    // unicodes we know that this will all complete without releasing
+    // the locks.
+    BEGIN_TYPE_DICT_LOCK(dict);
+
+    if (_PyDict_GetItemRef_Unicode_LockHeld((PyDictObject *)dict, name, &old_value) < 0) {
+        return -1;
+    }
+
+#ifdef Py_GIL_DISABLED
+    // In free-threaded builds readers can race with the lock-free portion
+    // of the type cache and the assignment into the dict.  We clear all of the
+    // versions initially so no readers will succeed in the lock-free case.
+    // They'll then block on the type lock until the update below is done.
+    type_modification_starting_unlocked(type);
+#endif
+
+    res = _PyDict_SetItem_LockHeld((PyDictObject *)dict, name, value);
+
+    /* Clear the VALID_VERSION flag of 'type' and all its
+        subclasses.  This could possibly be unified with the
+        update_subclasses() recursion in update_slot(), but carefully:
+        they each have their own conditions on which to stop
+        recursing into subclasses. */
+    type_modified_unlocked(type);
 
-    BEGIN_TYPE_LOCK()
-    res = _PyObject_GenericSetAttrWithDict((PyObject *)type, name, value, NULL);
     if (res == 0) {
-        /* Clear the VALID_VERSION flag of 'type' and all its
-           subclasses.  This could possibly be unified with the
-           update_subclasses() recursion in update_slot(), but carefully:
-           they each have their own conditions on which to stop
-           recursing into subclasses. */
-        type_modified_unlocked(type);
-
         if (is_dunder_name(name)) {
             res = update_slot(type, name);
         }
-        assert(_PyType_CheckConsistency(type));
     }
-    END_TYPE_LOCK()
+    else if (PyErr_ExceptionMatches(PyExc_KeyError)) {
+        PyErr_Format(PyExc_AttributeError,
+                        "type object '%.50s' has no attribute '%U'",
+                        ((PyTypeObject*)type)->tp_name, name);
 
+        _PyObject_SetAttributeErrorContext((PyObject *)type, name);
+    }
+
+    assert(_PyType_CheckConsistency(type));
+
+    END_TYPE_DICT_LOCK();
+done:
     Py_DECREF(name);
+    Py_XDECREF(descr);
+    Py_XDECREF(old_value);
     return res;
 }
 
@@ -9343,33 +9459,33 @@ _Py_slot_tp_getattr_hook(PyObject *self, PyObject *name)
     /* speed hack: we could use lookup_maybe, but that would resolve the
        method fully for each attribute lookup for classes with
        __getattr__, even when the attribute is present. So we use
-       _PyType_Lookup and create the method only when needed, with
+       _PyType_LookupRef and create the method only when needed, with
        call_attribute. */
-    getattr = _PyType_Lookup(tp, &_Py_ID(__getattr__));
+    getattr = _PyType_LookupRef(tp, &_Py_ID(__getattr__));
     if (getattr == NULL) {
         /* No __getattr__ hook: use a simpler dispatcher */
         tp->tp_getattro = _Py_slot_tp_getattro;
         return _Py_slot_tp_getattro(self, name);
     }
-    Py_INCREF(getattr);
     /* speed hack: we could use lookup_maybe, but that would resolve the
        method fully for each attribute lookup for classes with
        __getattr__, even when self has the default __getattribute__
-       method. So we use _PyType_Lookup and create the method only when
+       method. So we use _PyType_LookupRef and create the method only when
        needed, with call_attribute. */
-    getattribute = _PyType_Lookup(tp, &_Py_ID(__getattribute__));
+    getattribute = _PyType_LookupRef(tp, &_Py_ID(__getattribute__));
     if (getattribute == NULL ||
         (Py_IS_TYPE(getattribute, &PyWrapperDescr_Type) &&
          ((PyWrapperDescrObject *)getattribute)->d_wrapped ==
              (void *)PyObject_GenericGetAttr)) {
+        Py_XDECREF(getattribute);
         res = _PyObject_GenericGetAttrWithDict(self, name, NULL, 1);
         /* if res == NULL with no exception set, then it must be an
            AttributeError suppressed by us. */
         if (res == NULL && !PyErr_Occurred()) {
             res = call_attribute(self, getattr, name);
         }
-    } else {
+    }
-        Py_INCREF(getattribute);
+    else {
         res = call_attribute(self, getattribute, name);
         Py_DECREF(getattribute);
         if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
@@ -9476,7 +9592,7 @@ slot_tp_descr_get(PyObject *self, PyObject *obj, PyObject *type)
     PyTypeObject *tp = Py_TYPE(self);
     PyObject *get;
 
-    get = _PyType_Lookup(tp, &_Py_ID(__get__));
+    get = _PyType_LookupRef(tp, &_Py_ID(__get__));
     if (get == NULL) {
         /* Avoid further slowdowns */
         if (tp->tp_descr_get == slot_tp_descr_get)
@@ -9488,7 +9604,9 @@ slot_tp_descr_get(PyObject *self, PyObject *obj, PyObject *type)
     if (type == NULL)
         type = Py_None;
     PyObject *stack[3] = {self, obj, type};
-    return PyObject_Vectorcall(get, stack, 3, NULL);
+    PyObject *res = PyObject_Vectorcall(get, stack, 3, NULL);
+    Py_DECREF(get);
+    return res;
 }
 
 static int
@@ -10383,6 +10501,7 @@ update_one_slot(PyTypeObject *type, pytype_slotdef *p)
                 type->tp_flags &= ~Py_TPFLAGS_HAVE_VECTORCALL;
             }
         }
+        Py_DECREF(descr);
     } while ((++p)->offset == offset);
     if (specific && !use_generic)
         *ptr = specific;