inc_block_group_ro
ret = inc_block_group_ro(block_group, 0);
ret = inc_block_group_ro(bg, 0);
inc_block_group_ro(cache, 1);
inc_block_group_ro(cache, 1);
inc_block_group_ro(cache, 1);
ret = inc_block_group_ro(cache, 0);
ret = inc_block_group_ro(cache, 0);
ret = inc_block_group_ro(cache, 0);