Relaxed atomic is sufficient, add targetted test

Author: DinoV

Include/cpython/listobject.h

@@ -45,7 +45,7 @@ PyList_SET_ITEM(PyObject *op, Py_ssize_t index, PyObject *value) {
     assert(0 <= index);
     assert(index < list->allocated);
 #ifdef Py_GIL_DISABLED
-    _Py_atomic_store_ptr_release(&list->ob_item[index], value);
+    _Py_atomic_store_ptr_relaxed(&list->ob_item[index], value);
 #else
     list->ob_item[index] = value;
 #endif

Lib/test/test_free_threading/test_list.py

@@ -0,0 +1,38 @@
+import unittest
+
+from threading import Thread
+from unittest import TestCase
+
+class TestList(TestCase):
+    def test_racing_iter_append(self):
+
+        l = []
+        OBJECT_COUNT = 10000
+        def writer_func():
+            for i in range(OBJECT_COUNT):
+                l.append(i + OBJECT_COUNT)
+
+        def reader_func():
+            while True:
+                count = len(l)
+                for i, x in enumerate(l):
+                   self.assertEqual(x, i + OBJECT_COUNT)
+                if count == OBJECT_COUNT:
+                    break
+
+        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()
+        print('writer done')
+        for reader in readers:
+            reader.join()
+            print('reader done')
+
+if __name__ == "__main__":
+    unittest.main()