Re: [PATCH v2 28/36] block: add bdrv_set_backing_noperm() transaction action

[Vladimir Sementsov-Ogievskiy]

05.02.2021 19:26, Kevin Wolf wrote:
> Am 27.11.2020 um 15:45 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > Split out no-perm part of bdrv_set_backing_hd() as a separate
> > transaction action. Note the in case of existing BdrvChild we reuse it,
> > not recreate, just to do less actions.
> >
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>
> >   /*
> >    * Sets the bs->backing link of a BDS. A new reference is created; callers
> >    * which don't need their own reference any more must call bdrv_unref().
> >    */
> > -void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
> > -                         Error **errp)
> > +static int bdrv_set_backing_noperm(BlockDriverState *bs,
> > +                                   BlockDriverState *backing_bs,
> > +                                   GSList **tran, Error **errp)
> >   {
> > -    bool update_inherits_from = bdrv_chain_contains(bs, backing_hd) &&
> > -        bdrv_inherits_from_recursive(backing_hd, bs);
> > +    int ret = 0;
> > +    bool update_inherits_from = bdrv_chain_contains(bs, backing_bs) &&
> > +        bdrv_inherits_from_recursive(backing_bs, bs);
> > +    GSList *attach_tran = NULL;
> > +    BdrvSetBackingNoPermState *s;
> >   
> >       if (bdrv_is_backing_chain_frozen(bs, child_bs(bs->backing), errp)) {
> > -        return;
> > +        return -EPERM;
> >       }
> >   
> > -    if (backing_hd) {
> > -        bdrv_ref(backing_hd);
> > +    if (bs->backing && backing_bs) {
> > +        bdrv_replace_child_safe(bs->backing, backing_bs, tran);
>
> The old code with separate bdrv_unref_child() and then
> bdrv_attach_child() tried to make the AioContests of bs and backing_bs
> compatible by moving one of the nodes if necessary.
>
> bdrv_replace_child_safe() doesn't seem to do that, but it only asserts
> that both nodes are already in the same context.
>
> I see that iotest 245 doesn't crash, which I think it should if this
> were broken, but where does the switch happen now?

Hmm. Seems on path "if (bs->backing && backing_bs) {" we really miss aio 
context handling. Probably 245 doesn't check this branch? Or if leaves different aio contexts in one 
subtree..


--
Best regards,
Vladimir