list_entry(write_list->next, struct dm_buffer, write_list);
list_del(&b->write_list);
submit_io(b, WRITE, b->block, write_endio);
list_entry(write_list->next, struct dm_buffer, write_list);
list_del(&b->write_list);
submit_io(b, WRITE, b->block, write_endio);