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

[Kevin Wolf]

Am 06.02.2020 um 11:21 hat Max Reitz geschrieben:
> On 05.02.20 16:55, Kevin Wolf wrote:
> > 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.
> 
> I thought we guaranteed that loosening permissions never fails.
> 
> (Well, you know.  It may “leak” permissions, but we’d never get an error
> here so there’s nothing to handle anyway.)

This is what I meant. We ignore the failure (i.e. don't return an error),
but the result still isn't completely correct ("leaked" permissions).

> >    It would be cleaner to use check+abort instead of actually setting
> >    the new permission.
> 
> Oh.  Yes.  Maybe.  It does require more code, though, because I’d rather
> not use bdrv_check_update_perm() from here as-is.

I'm not saying you need to do it, just that it would be cleaner. :-)

> > 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.
> 
> True.  I tried to do it right by having a post-replace cleanup function,
> but after a while that was just going nowhere, really.  So I just went
> with what’s patch 13 here.
> 
> But isn’t 13 enough, actually?  It check can_replace right before
> replacing in a drained section.  I can’t imagine the permissions to
> change there.

Permissions are tied to file locks, so an external process can just grab
the locks in between. But if I understand correctly, all we try here is
to have an additional safeguard to prevent the user from doing stupid
things. So I guess not being 100% is fine as long as it's documented in
the code.

Kevin

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