From: Sam Gross <colesbury@gmail.com>
Date: Fri, 6 Mar 2026 17:01:06 +0000 (-0500)
Subject: gh-145566: Skip stop-the-world when reassigning `__class__` on newly created objects... 
X-Git-Tag: v3.15.0a7~40
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1d091a336e60b703a7d7ae4124f652eabe144f4e;p=thirdparty%2FPython%2Fcpython.git

gh-145566: Skip stop-the-world when reassigning `__class__` on newly created objects (gh-145567)
---

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-03-05-19-10-56.gh-issue-145566.H4RupyYN.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-05-19-10-56.gh-issue-145566.H4RupyYN.rst
new file mode 100644
index 000000000000..723b81ddc5f8
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-05-19-10-56.gh-issue-145566.H4RupyYN.rst
@@ -0,0 +1,2 @@
+In the free threading build, skip the stop-the-world pause when reassigning
+``__class__`` on a newly created object.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 1fdd3cbdaaa6..27ec8bb40a92 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -7568,7 +7568,11 @@ object_set_class_world_stopped(PyObject *self, PyTypeObject *newto)
 
             assert(_PyObject_GetManagedDict(self) == dict);
 
-            if (_PyDict_DetachFromObject(dict, self) < 0) {
+            int err;
+            Py_BEGIN_CRITICAL_SECTION(dict);
+            err = _PyDict_DetachFromObject(dict, self);
+            Py_END_CRITICAL_SECTION();
+            if (err < 0) {
                 return -1;
             }
 
@@ -7608,10 +7612,15 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
         return -1;
     }
 
-    types_stop_world();
+    int unique = _PyObject_IsUniquelyReferenced(self);
+    if (!unique) {
+        types_stop_world();
+    }
     PyTypeObject *oldto = Py_TYPE(self);
     int res = object_set_class_world_stopped(self, newto);
-    types_start_world();
+    if (!unique) {
+        types_start_world();
+    }
     if (res == 0) {
         if (oldto->tp_flags & Py_TPFLAGS_HEAPTYPE) {
             Py_DECREF(oldto);