mrb_write_barrier(mrb_state *mrb, struct RBasic *obj)
{
  mrb_gc *gc = &mrb->gc;

  if (!is_black(obj)) return;

  mrb_assert(!is_dead(gc, obj));
  mrb_assert(is_generational(gc) || gc->state != MRB_GC_STATE_ROOT);
  paint_gray(obj);
  obj->gcnext = gc->atomic_gray_list;
  gc->atomic_gray_list = obj;
}