On Fri, May 20, 2016 at 03:36:18PM +0800, Changlong Xie wrote:
> +        /* start backup job now */
> +        error_setg(&s->blocker,
> +                   "block device is in use by internal backup job");
> +
> +        top_bs = bdrv_lookup_bs(s->top_id, s->top_id, errp);
> +        if (!top_bs || !check_top_bs(top_bs, bs)) {
> +            reopen_backing_file(s, false, NULL);
> +            aio_context_release(aio_context);
> +            return;
> +        }

Missing error_setg() with an error message when check_top_bs() fails.