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Re: [Qemu-devel] [PATCH v2 06/12] block: Deep-clear inherits_from

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v2 06/12] block: Deep-clear inherits_from
Date: Wed, 17 Jul 2019 10:17:02 +0200
User-agent: Mutt/1.11.3 (2019-02-01)

Am 17.07.2019 um 09:47 hat Max Reitz geschrieben:
> On 16.07.19 19:01, Kevin Wolf wrote:
> > Am 03.07.2019 um 19:28 hat Max Reitz geschrieben:
> >> BDS.inherits_from does not always point to an immediate parent node.
> >> When launching a block job with a filter node, for example, the node
> >> directly below the filter will not point to the filter, but keep its old
> >> pointee (above the filter).
> >>
> >> If that pointee goes away while the job is still running, the node's
> >> inherits_from will not be updated and thus point to garbage.  To fix
> >> this, bdrv_unref_child() has to check not only the parent node's
> >> immediate children for nodes whose inherits_from needs to be cleared,
> >> but its whole subtree.
> >>
> >> Signed-off-by: Max Reitz <address@hidden>
> > 
> > Isn't the real bug that we keep pointing to a node that isn't a parent
> > of the node any more? I think this will effectively disable option
> > inheritance in bdrv_reopen() as long as the filter node is present,
> > which is certainly not what we intended.
> Well, it breaks it while a block job is running.  I don’t know whether I
> would advise doing a reopen across a block job filter.  It’s a case of
> “Why wouldn’t it work?”, but I’m sure there’s something that doesn’t.
> (Like this here, for example, but it at least has the decency of just
> letting the reopen fail.)

Why would it let the reopen fail? Failing would be justifiable, but I
expect it just succeeds without updating the options of the child node.

> > The intuitive thing would be that after inserting a filter, the image
> > now inherits from the filter node, and when the filter is removed, it
> > inherits from the filter's bs->inherit_from if that becomes a parent of
> > the node. (Though I'm not necessarily saying that my intuition is to be
> > trusted here.)
> I tried that first, but I couldn’t get it to work.  I don’t remember
> why, though.  I suppose my problem was that removing the filter node
> make inherits_from NULL.  I guess I stopped at that point and went this
> route instead.
> I suppose we could add a flag to the BDS that says whether an heir
> node’s inherits_from should be cleared or set to the bequeather’s
> inherits_from, like so:
>     if (parent->inherit_inherits_from) {
>         child->bs->inherits_from = parent->inherits_from;
>     } else {
>         child->bs->inherits_from = NULL;
>     }
> And then, if you insert a node between a child and its inherits_from
> parent, that node copies inherits_from from the child and gets its
> inherit_inherits_from set to true.
> The problem I see is that it doesn’t always appear clear to me that this
> intermediate node should actually copy its inherits_from from the child.
> So the same question applies here, too, I guess; should the filter node
> even inherit its options from the parent?

An explicitly created filter node that is inserted with blockdev-reopen
certainly doesn't inherit from its parent. An automatically created
filter node where you didn't have control of its options - hm... good

If we want to keep it simple, maybe it would be defensible if we just
break the inheritance relationship as soon as the parent is detached
(i.e. when inserting the filter). At least that would be more consistent
than silently disabling option inheritance while a filter is present and
then suddenly reenabling it when the filter goes away.

The other option would be making it actually work, i.e. the child node
keeps inheriting from its original parent node. This would not only
require modifications to bdrv_reopen(), but also to the data structures
so that the parent has a list of nodes that inherit from it: Nobody can
even guarantee that the child node always stays in the subtree of the
original parent.

This is in fact a reason why this patch isn't only ugly, but actually
wrong - you may still miss some inherits_from references.


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