gh-143100: Fix memcpy data race in setobject.c (gh-143127)

Author: colesbury

Objects/setobject.c

@@ -1420,6 +1420,17 @@ set_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     return make_new_set(type, NULL);
 }
 
+#ifdef Py_GIL_DISABLED
+static void
+copy_small_table(setentry *dest, setentry *src)
+{
+    for (Py_ssize_t i = 0; i < PySet_MINSIZE; i++) {
+        _Py_atomic_store_ptr_release(&dest[i].key, src[i].key);
+        _Py_atomic_store_ssize_relaxed(&dest[i].hash, src[i].hash);
+    }
+}
+#endif
+
 /* set_swap_bodies() switches the contents of any two sets by moving their
    internal data pointers and, if needed, copying the internal smalltables.
    Semantically equivalent to:
@@ -1462,8 +1473,13 @@ set_swap_bodies(PySetObject *a, PySetObject *b)
 
     if (a_table == a->smalltable || b_table == b->smalltable) {
         memcpy(tab, a->smalltable, sizeof(tab));
+#ifndef Py_GIL_DISABLED
         memcpy(a->smalltable, b->smalltable, sizeof(tab));
         memcpy(b->smalltable, tab, sizeof(tab));
+#else
+        copy_small_table(a->smalltable, b->smalltable);
+        copy_small_table(b->smalltable, tab);
+#endif
     }
 
     if (PyType_IsSubtype(Py_TYPE(a), &PyFrozenSet_Type)  &&

Tools/tsan/suppressions_free_threading.txt

@@ -20,6 +20,3 @@ thread:pthread_create
 # PyObject_Realloc internally does memcpy which isn't atomic so can race
 # with non-locking reads. See #132070
 race:PyObject_Realloc
-
-# gh-143100: set_swap_bodies in setobject.c calls memcpy, which isn't atomic
-race:set_swap_bodies