gh-129643: fix thread safety of `PyList_SetItem` (#129644)

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-02-04-12-42-40.gh-issue-129643.K24Zow.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-02-04-12-42-40.gh-issue-129643.K24Zow.rst
new file mode 100644 (file)
index 0000000..27dd3b7
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-02-04-12-42-40.gh-issue-129643.K24Zow.rst
@@ -0,0 +1 @@
+Fix thread safety of :c:func:`PyList_SetItem` in free-threading builds. Patch by Kumar Aditya.

diff --git a/Objects/listobject.c b/Objects/listobject.c
index f4a269e4d7b2849fbadd7d8a56003f3c14f86996..86fa21495564631cf29d12fb847ecffc87d4a7ec 100644 (file)
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -419,7 +419,6 @@ int
 PyList_SetItem(PyObject *op, Py_ssize_t i,
                PyObject *newitem)
 {
-    PyObject **p;
     if (!PyList_Check(op)) {
         Py_XDECREF(newitem);
         PyErr_BadInternalCall();
@@ -435,8 +434,9 @@ PyList_SetItem(PyObject *op, Py_ssize_t i,
         ret = -1;
         goto end;
     }
-    p = self->ob_item + i;
-    Py_XSETREF(*p, newitem);
+    PyObject *tmp = self->ob_item[i];
+    FT_ATOMIC_STORE_PTR_RELEASE(self->ob_item[i], newitem);
+    Py_XDECREF(tmp);
     ret = 0;
 end:;
     Py_END_CRITICAL_SECTION();