while ((n = rb_first(&td->io_hist_tree)) != NULL) {
ipo = rb_entry(n, struct io_piece, rb_node);
rb_erase(n, &td->io_hist_tree);
while ((n = rb_first(&td->io_hist_tree)) != NULL) {
ipo = rb_entry(n, struct io_piece, rb_node);
rb_erase(n, &td->io_hist_tree);