Re: [PATCH for-5.0 v2 10/23] quorum: Implement .bdrv_recurse_can_replace()

[Kevin Wolf]

Am 05.02.2020 um 16:55 hat Kevin Wolf geschrieben:
> Am 11.11.2019 um 17:02 hat Max Reitz geschrieben:
> > Signed-off-by: Max Reitz <address@hidden>
> > ---
> >  block/quorum.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 62 insertions(+)
> > 
> > diff --git a/block/quorum.c b/block/quorum.c
> > index 3a824e77e3..8ee03e9baf 100644
> > --- a/block/quorum.c
> > +++ b/block/quorum.c
> > @@ -825,6 +825,67 @@ static bool 
> > quorum_recurse_is_first_non_filter(BlockDriverState *bs,
> >      return false;
> >  }
> >  
> > +static bool quorum_recurse_can_replace(BlockDriverState *bs,
> > +                                       BlockDriverState *to_replace)
> > +{
> > +    BDRVQuorumState *s = bs->opaque;
> > +    int i;
> > +
> > +    for (i = 0; i < s->num_children; i++) {
> > +        /*
> > +         * We have no idea whether our children show the same data as
> > +         * this node (@bs).  It is actually highly likely that
> > +         * @to_replace does not, because replacing a broken child is
> > +         * one of the main use cases here.
> > +         *
> > +         * We do know that the new BDS will match @bs, so replacing
> > +         * any of our children by it will be safe.  It cannot change
> > +         * the data this quorum node presents to its parents.
> > +         *
> > +         * However, replacing @to_replace by @bs in any of our
> > +         * children's chains may change visible data somewhere in
> > +         * there.  We therefore cannot recurse down those chains with
> > +         * bdrv_recurse_can_replace().
> > +         * (More formally, bdrv_recurse_can_replace() requires that
> > +         * @to_replace will be replaced by something matching the @bs
> > +         * passed to it.  We cannot guarantee that.)
> > +         *
> > +         * Thus, we can only check whether any of our immediate
> > +         * children matches @to_replace.
> > +         *
> > +         * (In the future, we might add a function to recurse down a
> > +         * chain that checks that nothing there cares about a change
> > +         * in data from the respective child in question.  For
> > +         * example, most filters do not care when their child's data
> > +         * suddenly changes, as long as their parents do not care.)
> > +         */
> > +        if (s->children[i].child->bs == to_replace) {
> > +            Error *local_err = NULL;
> > +
> > +            /*
> > +             * We now have to ensure that there is no other parent
> > +             * that cares about replacing this child by a node with
> > +             * potentially different data.
> > +             */
> > +            s->children[i].to_be_replaced = true;
> > +            bdrv_child_refresh_perms(bs, s->children[i].child, &local_err);
> > +
> > +            /* Revert permissions */
> > +            s->children[i].to_be_replaced = false;
> > +            bdrv_child_refresh_perms(bs, s->children[i].child, 
> > &error_abort);
> 
> Quite a hack. The two obvious problems are:
> 
> 1. We can't guarantee that we can actually revert the permissions. I
>    think we ignore failure to loosen permissions meanwhile so that at
>    least the &error_abort doesn't trigger, but bs could still be in the
>    wrong state afterwards.
> 
>    It would be cleaner to use check+abort instead of actually setting
>    the new permission.
> 
> 2. As aborting the permission change makes more obvious, we're checking
>    something that might not be true any more when we actually make the
>    change.
> 
> Pragmatically, a hack might be good enough here, but it should be
> documented as such (with a short explanation of its shortcomings) at
> least.

Oops, meant to send this as a comment for v3 (which I did apply locally
for review).

Kevin