gh-117657: Fix TSAN list set failure (#118260)

* Fix TSAN list set failure

* Relaxed atomic is sufficient, add targetted test

* More list

* Remove atomic assign in list

* Fixup white space

Lib/test/test_free_threading/test_list.py

@@ -0,0 +1,80 @@
+import unittest
+
+from threading import Thread
+from unittest import TestCase
+
+from test.support import is_wasi
+
+
+class C:
+    def __init__(self, v):
+        self.v = v
+
+
+@unittest.skipIf(is_wasi, "WASI has no threads.")
+class TestList(TestCase):
+    def test_racing_iter_append(self):
+
+        l = []
+        OBJECT_COUNT = 10000
+
+        def writer_func():
+            for i in range(OBJECT_COUNT):
+                l.append(C(i + OBJECT_COUNT))
+
+        def reader_func():
+            while True:
+                count = len(l)
+                for i, x in enumerate(l):
+                    self.assertEqual(x.v, 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()
+        for reader in readers:
+            reader.join()
+
+    def test_racing_iter_extend(self):
+        iters = [
+            lambda x: [x],
+        ]
+        for iter_case in iters:
+            with self.subTest(iter=iter_case):
+                l = []
+                OBJECT_COUNT = 10000
+
+                def writer_func():
+                    for i in range(OBJECT_COUNT):
+                        l.extend(iter_case(C(i + OBJECT_COUNT)))
+
+                def reader_func():
+                    while True:
+                        count = len(l)
+                        for i, x in enumerate(l):
+                            self.assertEqual(x.v, 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()
+                for reader in readers:
+                    reader.join()
+
+
+if __name__ == "__main__":
+    unittest.main()