Re: No way to break bio_for_each_segment_all() macro?

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Apr 06, 2019 at 09:53:07AM +0800, Qu Wenruo wrote:
> Hi,
> 
> I'm looking into a strange behavior that we can't break
> bio_for_each_segment_all() after commit 6dc4f100c175 ("block: allow
> bio_for_each_segment_all() to iterate over multi-page bvec").
> 
> It's screwing up all bio_for_each_segment_all() call with error out branch.

>         bio_for_each_segment_all(bvec, bio, i, iter_all) {
>                 root = BTRFS_I(bvec->bv_page->mapping->host)->root;
>                 ret = csum_dirty_buffer(root->fs_info, bvec->bv_page);
> -               if (ret)
> +               if (ret) {
> +                       err = ret;
> +                       pr_info("breaking out with ret=%d\n", ret);
>                         break;
> +               }
>         }
> 
> +       if (err)
> +               pr_info("err=%d out, but ret=%d\n",err, ret);
>         return errno_to_blk_status(ret);
>  }
> 
> Straightforward, if we break, we should have err == ret.
> 
> Then run fstests btrfs/151, which will trigger a false alert in
> tree-checker:
> 
>   BTRFS critical (device dm-1): corrupt leaf: root=3 block=570572800
> slot=1 devid=1 invalid total bytes: have 0
>   BTRFS error (device dm-1): block=570572800 write time tree block
> corruption detected
>   breaking out with ret=-117
>   err=-117 out, but ret=0
> 
> So it looks like the break line doens't really break, but continue
> executing.

It expands to for-inside-for since that commit, so break only takes you
out of the inner loop...

No comments on desirability of such macros - personally, I prefer to
avoid those, but I'm not stepping into that holy war...