From ebd98bd8ed0dc7e689b84d76e673363b4bae1895 Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Wed, 8 Jul 2026 13:23:43 +0300
Subject: [PATCH 1/2] gh-153298: Fix data race in `GenericAlias` parameter
 initialization on FT

---
 Lib/test/test_free_threading/test_types.py    | 34 +++++++++++++++++++
 ...-07-08-13-23-11.gh-issue-153298.wvcXxN.rst |  2 ++
 Objects/genericaliasobject.c                  | 12 ++++++-
 3 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 Lib/test/test_free_threading/test_types.py
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-07-08-13-23-11.gh-issue-153298.wvcXxN.rst

diff --git a/Lib/test/test_free_threading/test_types.py b/Lib/test/test_free_threading/test_types.py
new file mode 100644
index 000000000000000..84375515168fdce
--- /dev/null
+++ b/Lib/test/test_free_threading/test_types.py
@@ -0,0 +1,34 @@
+import unittest
+import types
+from typing import TypeVar
+from test.support import threading_helper
+
+threading_helper.requires_working_threading(module=True)
+
+
+class TestGenericAlias(unittest.TestCase):
+    def test_repr_race(self):
+        # gh-153298
+
+        T = TypeVar('T')
+        slot = [list[T]]
+
+        def access():
+            for _ in range(2000):
+                try:
+                    _ = slot[0].__parameters__
+                except Exception:
+                    pass
+
+        def refresh():
+            for _ in range(2000):
+                slot[0] = list[T]
+
+        threading_helper.run_concurrently([
+            *[access for _ in range(6)],
+            *[refresh for _ in range(2)],
+        ])
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-07-08-13-23-11.gh-issue-153298.wvcXxN.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-08-13-23-11.gh-issue-153298.wvcXxN.rst
new file mode 100644
index 000000000000000..38d548834d5e02e
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-08-13-23-11.gh-issue-153298.wvcXxN.rst
@@ -0,0 +1,2 @@
+Fixes a data race in :class:`types.GenericAlias` ``__parameters__``
+initialization on free-threading builds.
diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index c2083e6fcb77f47..4e85927def7ea78 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -841,7 +841,7 @@ static PyMemberDef ga_members[] = {
 };
 
 static PyObject *
-ga_parameters(PyObject *self, void *unused)
+ga_parameters_lock_held(PyObject *self)
 {
     gaobject *alias = (gaobject *)self;
     if (alias->parameters == NULL) {
@@ -853,6 +853,16 @@ ga_parameters(PyObject *self, void *unused)
     return Py_NewRef(alias->parameters);
 }
 
+static PyObject *
+ga_parameters(PyObject *self, void *unused)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(self);
+    result = ga_parameters_lock_held(self);
+    Py_END_CRITICAL_SECTION();
+    return result;
+}
+
 static PyObject *
 ga_unpacked_tuple_args(PyObject *self, void *unused)
 {

From 059d1f5a9793519842fa4c139ac8f211d1f4cd5b Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Wed, 8 Jul 2026 13:37:22 +0300
Subject: [PATCH 2/2] fix ci

---
 Lib/test/test_free_threading/test_types.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/Lib/test/test_free_threading/test_types.py b/Lib/test/test_free_threading/test_types.py
index 84375515168fdce..76fcf1590122f54 100644
--- a/Lib/test/test_free_threading/test_types.py
+++ b/Lib/test/test_free_threading/test_types.py
@@ -1,5 +1,4 @@
 import unittest
-import types
 from typing import TypeVar
 from test.support import threading_helper
 
@@ -7,7 +6,7 @@
 
 
 class TestGenericAlias(unittest.TestCase):
-    def test_repr_race(self):
+    def test_parameters_race(self):
         # gh-153298
 
         T = TypeVar('T')