Incref dict when it's usage is borrowed

Lib/test/test_free_threading/test_dict.py

@@ -142,6 +142,49 @@ def writer_func(l):
             for ref in thread_list:
                 self.assertIsNone(ref())
 
+    def test_racing_set_object_dict(self):
+        """Races assigning to __dict__ should be thread safe"""
+
+        class C: pass
+        f = C()
+        f.__dict__ = {"foo": 42}
+        THREAD_COUNT = 10
+        class MyDict(dict): pass
+
+        def writer_func():
+            for i in range(100000):
+                f.__dict__ = {"foo": 100}
+
+        def reader_func():
+            for i in range(100000):
+                f.foo
+
+        lists = []
+        readers = []
+        writers = []
+        for x in range(THREAD_COUNT):
+            thread_list = []
+            lists.append(thread_list)
+            writer = Thread(target=partial(writer_func))
+            writers.append(writer)
+
+        for x in range(THREAD_COUNT):
+            thread_list = []
+            lists.append(thread_list)
+            reader = Thread(target=partial(reader_func))
+            readers.append(reader)
+
+        for writer in writers:
+            writer.start()
+        for reader in readers:
+            reader.start()
+
+        for writer in writers:
+            writer.join()
+
+        for reader in readers:
+            reader.join()
+
     @unittest.skipIf(_testcapi is None, 'need _testcapi module')
     def test_dict_version(self):
         dict_version = _testcapi.dict_version

Objects/dictobject.c

@@ -7082,6 +7082,7 @@ _PyObject_SetManagedDict(PyObject *obj, PyObject *new_dict)
     PyTypeObject *tp = Py_TYPE(obj);
     if (tp->tp_flags & Py_TPFLAGS_INLINE_VALUES) {
         PyDictObject *dict = _PyObject_GetManagedDict(obj);
+        PyDictObject *locked_dict;
         if (dict == NULL) {
 #ifdef Py_GIL_DISABLED
             Py_BEGIN_CRITICAL_SECTION(obj);
@@ -7091,6 +7092,9 @@ _PyObject_SetManagedDict(PyObject *obj, PyObject *new_dict)
                 set_dict_inline_values(obj, (PyDictObject *)new_dict);
             }
 
+            Py_XINCREF(dict);
+            locked_dict = dict;
+
             Py_END_CRITICAL_SECTION();
 
             if (dict == NULL) {
@@ -7102,6 +7106,21 @@ _PyObject_SetManagedDict(PyObject *obj, PyObject *new_dict)
 #endif
         }
 
+#ifdef Py_GIL_DISABLED
+        else {
+            // We only have a borrowed reference to the dict, so we
+            // need to incref it.
+            locked_dict = dict;
+            if (!_Py_TryIncref((PyObject *)locked_dict)) {
+                Py_BEGIN_CRITICAL_SECTION(obj);
+                locked_dict = _PyObject_GetManagedDict(obj);
+                Py_INCREF(locked_dict);
+                dict = locked_dict;
+                Py_END_CRITICAL_SECTION();
+            }
+        }
+#endif
+
         Py_BEGIN_CRITICAL_SECTION2(dict, obj);
 
         // We've locked dict, but the actual dict could have changed
@@ -7115,7 +7134,10 @@ _PyObject_SetManagedDict(PyObject *obj, PyObject *new_dict)
         Py_END_CRITICAL_SECTION2();
 
         if (err == 0) {
-            Py_XDECREF(dict);
+#ifdef Py_GIL_DISABLED
+            Py_DECREF(locked_dict); // Reference for keeping dict alive
+#endif
+            Py_XDECREF(dict);       // Reference we replaced in object
         }
     }
     else {

Objects/object.c

@@ -1,4 +1,3 @@
-
 /* Generic object operations; and implementation of None */
 
 #include "Python.h"
@@ -1618,7 +1617,21 @@ _PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name,
         }
     }
     if (dict != NULL) {
+#ifdef Py_GIL_DISABLED
+        if (!_Py_TryIncref(dict)) {
+            Py_BEGIN_CRITICAL_SECTION(obj);
+            if ((tp->tp_flags & Py_TPFLAGS_MANAGED_DICT)) {
+                dict = (PyObject *)_PyObject_GetManagedDict(obj);
+            }
+            else {
+                dict = *_PyObject_ComputedDictPointer(obj);
+            }
+            Py_INCREF(dict);
+            Py_END_CRITICAL_SECTION();
+        }
+#else
         Py_INCREF(dict);
+#endif
         int rc = PyDict_GetItemRef(dict, name, &res);
         Py_DECREF(dict);
         if (res != NULL) {