gh-155628: Use a relaxed add for the dict shared-keys incref

An INCREF_KEYS may only run while the caller already keeps the keys
alive: every dictkeys_incref call site holds either the dict's
per-object lock while dk is its ma_keys (which is also required to
replace ma_keys), or a strong reference chain such as the type's
cached keys. The increment therefore cannot race with the final
decref and needs no ordering. DECREF_KEYS is unchanged: the final
decref must order the keys' contents before freeing them.

Author: kddnewton

Objects/dictobject.c

@@ -230,8 +230,10 @@ set_values(PyDictObject *mp, PyDictValues *values)
 #define ASSERT_KEYS_LOCKED(keys) assert(PyMutex_IsLocked(&keys->dk_mutex))
 #define LOAD_SHARED_KEY(key) _Py_atomic_load_ptr_acquire(&key)
 #define STORE_SHARED_KEY(key, value) _Py_atomic_store_ptr_release(&key, value)
-// Inc refs the keys object, giving the previous value
-#define INCREF_KEYS(dk)  _Py_atomic_add_ssize(&dk->dk_refcnt, 1)
+// Inc refs the keys object, giving the previous value. Relaxed ordering
+// suffices: an incref may only run while the caller already keeps the keys
+// alive, and the final decref provides the ordering needed to free them.
+#define INCREF_KEYS(dk)  _Py_atomic_add_ssize_relaxed(&dk->dk_refcnt, 1)
 // Dec refs the keys object, giving the previous value
 #define DECREF_KEYS(dk)  _Py_atomic_add_ssize(&dk->dk_refcnt, -1)
 #define LOAD_KEYS_NENTRIES(keys) _Py_atomic_load_ssize_relaxed(&keys->dk_nentries)