gh-129643: Fix `PyList_Insert` in free-threading builds (#129680)

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-02-05-11-29-52.gh-issue-129643.4mGzvg.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-02-05-11-29-52.gh-issue-129643.4mGzvg.rst
new file mode 100644 (file)
index 0000000..420e1fb
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-02-05-11-29-52.gh-issue-129643.4mGzvg.rst
@@ -0,0 +1 @@
+Fix thread safety of :c:func:`PyList_Insert` in free-threading builds.

diff --git a/Objects/listobject.c b/Objects/listobject.c
index 86fa21495564631cf29d12fb847ecffc87d4a7ec..120e353b709e7bfbaf6dbe4d1bdfc63fae11b744 100644 (file)
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -466,8 +466,8 @@ ins1(PyListObject *self, Py_ssize_t where, PyObject *v)
         where = n;
     items = self->ob_item;
     for (i = n; --i >= where; )
-        items[i+1] = items[i];
-    items[where] = Py_NewRef(v);
+        FT_ATOMIC_STORE_PTR_RELAXED(items[i+1], items[i]);
+    FT_ATOMIC_STORE_PTR_RELEASE(items[where], Py_NewRef(v));
     return 0;
 }