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

[Max Reitz]

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.)

>    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.

> 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.

Max

> Pragmatically, a hack might be good enough here, but it should be
> documented as such (with a short explanation of its shortcomings) at
> least.

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