gh-145036: Fix data race for list capacity in free-threading (#145365)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/test/test_free_threading/test_list.py

@@ -149,6 +149,27 @@ def reader_list(b, l):
         with threading_helper.start_threads(threads):
             pass
 
+    # gh-145036: race condition with list.__sizeof__()
+    def test_list_sizeof_free_threaded_build(self):
+        L = []
+
+        def mutate_function():
+            for _ in range(100):
+                L.append(1)
+                L.pop()
+
+        def size_function():
+            for _ in range(100):
+                L.__sizeof__()
+
+        threads = []
+        for _ in range(4):
+            threads.append(Thread(target=mutate_function))
+            threads.append(Thread(target=size_function))
+
+        with threading_helper.start_threads(threads):
+            pass
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-02-28-18-42-36.gh-issue-145036.70Kbfz.rst

@@ -0,0 +1 @@
+In free-threaded build, fix race condition when calling :meth:`!__sizeof__` on a :class:`list`

Objects/listobject.c

@@ -3582,8 +3582,14 @@ list___sizeof___impl(PyListObject *self)
 /*[clinic end generated code: output=3417541f95f9a53e input=b8030a5d5ce8a187]*/
 {
     size_t res = _PyObject_SIZE(Py_TYPE(self));
-    Py_ssize_t allocated = FT_ATOMIC_LOAD_SSIZE_RELAXED(self->allocated);
-    res += (size_t)allocated * sizeof(void*);
+#ifdef Py_GIL_DISABLED
+    PyObject **ob_item = _Py_atomic_load_ptr(&self->ob_item);
+    if (ob_item != NULL) {
+        res += list_capacity(ob_item) * sizeof(PyObject *);
+    }
+#else
+    res += (size_t)self->allocated * sizeof(PyObject *);
+#endif
     return PyLong_FromSize_t(res);
 }