gh-154044: Fix data race on descriptor __qualname__ in FT build

Lib/test/test_free_threading/test_descr.py

@@ -0,0 +1,46 @@
+import unittest
+
+from test import support
+from test.support import threading_helper
+
+
+N = 8
+
+
+@threading_helper.requires_working_threading()
+class TestDescrQualnameRace(unittest.TestCase):
+    # gh-154044: reading __qualname__ on a shared descriptor for the first time
+    # concurrently raced on the lazy d_qualname cache.
+
+    def race_first_access(self, descr):
+        results = []
+
+        def read():
+            results.append(descr.__qualname__)
+
+        threading_helper.run_concurrently(read, N)
+        self.assertEqual(len(set(results)), 1)
+        self.assertIsNotNone(results[0])
+
+    def test_slot_member_descriptors(self):
+        count = 100 if support.check_sanitizer(thread=True) else 300
+        for _ in range(count):
+            class C:
+                __slots__ = ("value",)
+            self.race_first_access(C.__dict__["value"])
+
+    def test_builtin_descriptors(self):
+        kinds = {"method_descriptor", "getset_descriptor", "wrapper_descriptor"}
+        descrs = [
+            v
+            for tp in (str, bytes, list, dict, set, int, float, tuple,
+                       frozenset, bytearray)
+            for v in vars(tp).values()
+            if type(v).__name__ in kinds
+        ]
+        for descr in descrs:
+            self.race_first_access(descr)
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-07-25-12-44-13.gh-issue-154044.GBa2wP.rst

@@ -0,0 +1,2 @@
+Fix a data race on a descriptor's ``__qualname__`` cache in the
+:term:`free-threaded build`.

Objects/descrobject.c

@@ -9,6 +9,7 @@
 #include "pycore_modsupport.h"    // _PyArg_UnpackStack()
 #include "pycore_object.h"        // _PyObject_GC_UNTRACK()
 #include "pycore_object_deferred.h" // _PyObject_SetDeferredRefcount()
+#include "pycore_pyatomic_ft_wrappers.h" // FT_ATOMIC_LOAD_PTR_ACQUIRE()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 #include "pycore_tuple.h"         // _PyTuple_ITEMS()
 
@@ -621,9 +622,24 @@ static PyObject *
 descr_get_qualname(PyObject *self, void *Py_UNUSED(ignored))
 {
     PyDescrObject *descr = (PyDescrObject *)self;
-    if (descr->d_qualname == NULL)
-        descr->d_qualname = calculate_qualname(descr);
-    return Py_XNewRef(descr->d_qualname);
+
+    PyObject *qualname = FT_ATOMIC_LOAD_PTR_ACQUIRE(descr->d_qualname);
+    if (qualname != NULL) {
+        return Py_NewRef(qualname);
+    }
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    qualname = FT_ATOMIC_LOAD_PTR_RELAXED(descr->d_qualname);
+    if (qualname == NULL) {
+        qualname = calculate_qualname(descr);
+        if (qualname != NULL) {
+            FT_ATOMIC_STORE_PTR_RELEASE(descr->d_qualname, qualname);
+        }
+    }
+    Py_XINCREF(qualname);
+    Py_END_CRITICAL_SECTION();
+
+    return qualname;
 }
 
 static PyObject *