Similar to the bug in GMainLoop, the eventfd changes to GCancellable do this:

#ifdef HAVE_EVENTFD
  priv->cancel_pipe[0] = eventfd (0, EFD_CLOEXEC | EFD_NONBLOCK);
  if (priv->cancel_pipe[0] >= 0)
    {
      if (priv->cancelled)
        g_cancellable_write_cancelled (cancellable);
      return;
    }
  else if (errno != ENOSYS)
    return;
  /* Fall through on ENOSYS */
#endif


without dealing with the EINVAL case caused by using EFD_CLOEXEC on some old versions of Linux.