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

[Alberto Garcia]

On Thu 14 Feb 2019 04:52:52 PM CET, Kevin Wolf wrote:
> Am 14.02.2019 um 15:13 hat Alberto Garcia geschrieben:
>> On Tue 12 Feb 2019 03:47:47 PM CET, Kevin Wolf wrote:
>> > Am 17.01.2019 um 16:33 hat Alberto Garcia geschrieben:
>> >> @@ -3861,6 +3923,10 @@ int bdrv_drop_intermediate(BlockDriverState *top, 
>> >> BlockDriverState *base,
>> >>          goto exit;
>> >>      }
>> >>  
>> >> +    if (bdrv_is_backing_chain_frozen(top, base, NULL)) {
>> >> +        goto exit;
>> >> +    }
>> >> +
>> >>      /* If 'base' recursively inherits from 'top' then we should set
>> >>       * base->inherits_from to top->inherits_from after 'top' and all
>> >>       * other intermediate nodes have been dropped.
>> >
>> > bdrv_drop_intermediate() doesn't change the links between in the chain
>> > between top and base, but the links between the parents of top and top
>> > are changed to point to base instead.
>> >
>> > I think this is checking the wrong thing.
>> 
>> You're right, those links should be checked.
>> 
>> On the other hand it does remove all references from top's parents to
>> top, so in the general case it will end up deleting all intermediate
>> notes, and their backing links with them. So I think we still need that
>> check.
>
> That begs the question if removing a frozen child link should be
> forbideen even when it's because the parent goes away. It don't think
> there is a good reason to check this - who else than the removed
> parent could be interested in keeping it frozen?
>
> In fact, this means that the parent forgot to unfreeze its child
> before going away, which shouldn't happen. So I guess bdrv_close()
> could just assert that this is not the case for any children?

That actually sounds like a good solution.

Berto