Re: [Qemu-block] [Qemu-devel] [PATCH v2 01/13] block: Allow freezing BdrvChild links

[Alberto Garcia]

On Wed 13 Mar 2019 03:29:36 PM CET, Kevin Wolf wrote:
> Am 13.03.2019 um 15:14 hat Alberto Garcia geschrieben:
>> On Tue 12 Mar 2019 05:12:53 PM CET, Vladimir Sementsov-Ogievskiy wrote:
>> >> +    /* This function changes all links that point to top and makes
>> >> +     * them point to base. Check that none of them is frozen. */
>> >> +    QLIST_FOREACH(c, &top->parents, next_parent) {
>> >> +        if (c->frozen) {
>> >> +            goto exit;
>> >> +        }
>> >> +    }
>> >> +
>> 
>> > Hmm, looking at this I have a bit unrelated questions about
>> > bdrv_drop_intermediate:
>> >
>> > 1. if we fail on some iteration of QLIST_FOREACH_SAFE loop, we finish
>> > up with partly updated graph??
>> 
>> >    I don't see any kind of roll-back in this case.. Why it don't
>> >    operate like bdrv_replace_node, which firstly check all
>> >    permissions, and then update all parents in fail-free loop?
>> 
>> I think you're right. If you don't have something written already I
>> could try to do it.
>> 
>> > 2. And therefore, why we just don't call bdrv_replace_node(top, base,
>> > errp) from bdrv_drop_intermediate?
>> 
>> I think that would not call role->update_filename().
>
> And role->update_filename() involves I/O, so you can't roll back
> across it anyway. I think this was the reason why we didn't roll back
> the in-memory state either, it would become inconsistent with what is
> on the disk.

But I think at least for the bdrv_check_update_perm() case we should
call bdrv_abort_perm_update() on all nodes if things fail, and we're
currently not doing it.

Berto