Re: Xen 4.18 release: Reminder about code freeze

[George Dunlap <george.dunlap@xxxxxxxxx>]

On Thu, Oct 12, 2023 at 11:36 PM Stefano Stabellini
<sstabellini@xxxxxxxxxx> wrote:
>
> On Thu, 12 Oct 2023, George Dunlap wrote:
> > > > Stop tinkering in the hope that it hides the problem.  You're only
> > > > making it harder to fix properly.
> > >
> > > Making it harder to fix properly would be a valid reason not to commit
> > > the (maybe partial) fix. But looking at the fix again:
> > >
> > > diff --git a/tools/xenstored/domain.c b/tools/xenstored/domain.c
> > > index a6cd199fdc..9cd6678015 100644
> > > --- a/tools/xenstored/domain.c
> > > +++ b/tools/xenstored/domain.c
> > > @@ -989,6 +989,7 @@ static struct domain *introduce_domain(const void 
> > > *ctx,
> > >                 talloc_steal(domain->conn, domain);
> > >
> > >                 if (!restore) {
> > > +                       domain_conn_reset(domain);
> > >                         /* Notify the domain that xenstore is available */
> > >                         interface->connection = XENSTORE_CONNECTED;
> > >                         xenevtchn_notify(xce_handle, domain->port);
> > > @@ -1031,8 +1032,6 @@ int do_introduce(const void *ctx, struct connection 
> > > *conn,
> > >         if (!domain)
> > >                 return errno;
> > >
> > > -       domain_conn_reset(domain);
> > > -
> > >         send_ack(conn, XS_INTRODUCE);
> > >
> > > It is a 1-line movement. Textually small. Easy to understand and to
> > > revert. It doesn't seem to be making things harder to fix? We could
> > > revert it any time if a better fix is offered.
> > >
> > > Maybe we could have a XXX note in the commit message or in-code
> > > comment?
> >
> > It moves a line from one function (do_domain_introduce()) into a
> > completely different function (introduce_domain()), nested inside two
> > if() statements; with no analysis on how the change will impact
> > things.
>
> I am not the original author of the patch, and I am not the maintainer
> of the code, so I don't feel I have the qualifications to give you the
> answers you are seeking. Julien as author of the patch and xenstore
> reviewer might be in a better position to answer. Or Juergen as xenstore
> maintainer.

I understand that; my main point is that the change is more complex
than you're characterizing it.  This is information necessary to
understand whether the patch is correct, but it's not in the patch
description, nor in the subsequent thread back in May.

> > Are there any paths through do_domain_introduce() that now *won't* get
> > a domain_conn_reset() call?  Is that OK?
>
> Yes, the already-introduced and the restore code paths. The operations in
> the already-introduced or the restore code paths seem simple enough not
> to require a domain_conn_reset. Julien and Juergen should confirm.

There is no "restore" codepath through do_domain_introduce(); it
passes "false" for the "restore" argument.  So we  only have two paths
to consider through do_domain_introduce(): The "not introduced and not
restoring" path, and the "already-introduced" path.

I'm not sure what the "simple" elements on the branch in
introduce_domain() have to do with whether the content of the page
needs to be cleaned up.  As I said, I don't 100% understand this code,
but it seems like if anything, the reset would be *more* important to
have in the "reintroduce" case than in the "initial introduction"
case, since I'd expect the "initial introduction" case to be empty
already.

> Doesn't it seem weird to you that we set a connection to CONNECTED,
> notify the domain that it is ready to go, and only *after* that we reset
> the connection to zero?
>
> What happens if a domain starts using the connection as soon as it
> receives the event channel notification and before domain_conn_reset is
> called?

Yes, it does seem weird, which is why I said the following. :-)

> > I mean, it certainly seems strange to set the state to CONNECTED, send
> > off an event channel, and then after that delete all watches /
> > transactions / buffered data and so on;

But just because the current code is probably wrong, doesn't mean that
the modified code is probably correct.

If the problem is the delay between the xenevtchn_notify() in
introduce_domain() and the domain_conn_reset() afterwards in
do_domain(), would it make sense instead to move the notification into
do_introduce(), after the domain_conn_reset()?  It is, after all, in
response to XS_INTRODUCE that we want to send the notification, not in
dom0_init() or read_state_connection() (which seems to be more about
restoring a domain).

What about something like the attached? (NB not even compile tested)

 -George

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