Re: Plans to bring QMP 'x-blockdev-reopen' out of experimental?

[Alberto Garcia]

On Wed 02 Dec 2020 05:28:08 PM CET, Kevin Wolf wrote:

>> So x-blockdev-reopen sees that we want to replace the current
>> bs->file ("hd0-file") with a new one ("throttle0"). The problem here
>> is that throttle0 has hd0-file as its child, so when we check the
>> permissions on throttle0 (and its children) we get that hd0-file
>> refuses because it's already being used (although in in the process
>> of being replaced) by hd0:
>> 
>> "Conflicts with use by hd0 as 'file', which does not allow 'write, resize' 
>> on hd0-file"
>> 
> This kind of situation isn't new, I believe some of the existing graph
> changes (iirc in the context of block jobs) can cause the same problem.
>
> This is essentially why some functions in the permission system take a
> GSList *ignore_children. So I believe the right thing to do here is
> telling the permission system that it needs to check the situation
> without the BdrvChild that links hd0 with hd0-file.

I had tried this already and it does work when inserting the filter (we
know that 'hd0-file' is about to be detached from the parent so we can
put it in the list) but I don't think it's so easy if we want to remove
the filter, i.e.

   hd0 -> throttle -> hd0-file     ======>     hd0 -> hd0-file

In this case we get a similar error, we want to make hd0-file a child of
hd0 but it is being used by the throttle filter.

Telling bdrv_check_update_perm() to ignore hd0's current child
(throttle) won't solve the problem.

> I don't know the exact stack trace of your failure, so maybe this
> parameter isn't available yet in the place where you need it, but in
> the core functions it exists.

This is in bdrv_reopen_multiple(), in the same place where we are
currently checking the permissions of the new backing file.

Berto