Re: [PATCH 1/1] yank: Unregister function when using TLS migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Thu, May 27, 2021 at 09:09:09AM -0400, Peter Xu wrote:
> > On Thu, May 27, 2021 at 01:37:42PM +0100, Daniel P. Berrangé wrote:
> > > On Thu, May 27, 2021 at 08:23:52AM -0400, Peter Xu wrote:
> > > > On Thu, May 27, 2021 at 09:46:54AM +0100, Daniel P. Berrangé wrote:
> > > > > On Wed, May 26, 2021 at 05:58:58PM -0400, Peter Xu wrote:
> > > > > > On Wed, May 26, 2021 at 11:21:03PM +0200, Lukas Straub wrote:
> > > > > > > On Wed, 26 May 2021 16:40:35 -0400
> > > > > > > Peter Xu <peterx@redhat.com> wrote:
> > > > > > > 
> > > > > > > > On Wed, May 26, 2021 at 05:05:40PM -0300, Leonardo Bras wrote:
> > > > > > > > > After yank feature was introduced, whenever migration is started using TLS,
> > > > > > > > > the following error happens in both source and destination hosts:
> > > > > > > > > 
> > > > > > > > > (qemu) qemu-kvm: ../util/yank.c:107: yank_unregister_instance:
> > > > > > > > > Assertion `QLIST_EMPTY(&entry->yankfns)' failed.
> > > > > > > > > 
> > > > > > > > > This happens because of a missing yank_unregister_function() when using
> > > > > > > > > qio-channel-tls.
> > > > > > > > > 
> > > > > > > > > Fix this by also allowing TYPE_QIO_CHANNEL_TLS object type to perform
> > > > > > > > > yank_unregister_function() in channel_close() and multifd_load_cleanup().
> > > > > > > > > 
> > > > > > > > > Fixes: 50186051f ("Introduce yank feature")
> > > > > > > > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1964326
> > > > > > > > > Signed-off-by: Leonardo Bras <leobras.c@gmail.com>  
> > > > > > > > 
> > > > > > > > Leo,
> > > > > > > > 
> > > > > > > > Thanks for looking into it!
> > > > > > > > 
> > > > > > > > So before looking int the fix... I do have a doubt on why we only enable yank
> > > > > > > > on socket typed, as I think tls should also work with qio_channel_shutdown().
> > > > > > > > 
> > > > > > > > IIUC the confused thing here is we register only for qio-socket, however tls
> > > > > > > > will actually call migration_channel_connect() twice, first with a qio-socket,
> > > > > > > > then with the real tls-socket.  For tls I feel like we have registered with the
> > > > > > > > wrong channel - instead of the wrapper socket ioc, we should register to the
> > > > > > > > final tls ioc?
> > > > > > > > 
> > > > > > > > Lukas, is there a reason?
> > > > > > > > 
> > > > > > > 
> > > > > > > Hi,
> > > > > > > There is no specific reason. Both ways work equally well in preventing
> > > > > > > qemu from hanging. shutdown() for tls-channel just makes it abort a
> > > > > > > little sooner (by not attempting to encrypt and send data anymore).
> > > > > > > 
> > > > > > > I don't lean either way. I guess registering it on the tls-channel
> > > > > > > makes is a bit more explicit and clearer.
> > > > > > 
> > > > > > Agreed, because IMHO logically the migration code should not be aware of
> > > > > > internals of IOChannels, e.g., we shouldn't need to know ioc->master is the
> > > > > > socket ioc of tls ioc to unregister.
> > > > > 
> > > > > I think it is atually better to ignore the TLS channel and *always* yank
> > > > > on the undering socket IO channel. The yank functionality is intended to
> > > > > be used in a scenario where we know the channels are broken.  If yank
> > > > > calls the high level IO channel it is potentially going to try to do a
> > > > > cleanup shutdown that we know will fail because of the broken network.
> > > > 
> > > > Could you elaborate what's the "cleanup shutdown"?
> > > > 
> > > > The yank calls migration_yank_iochannel:
> > > > 
> > > > void migration_yank_iochannel(void *opaque)
> > > > {
> > > >     QIOChannel *ioc = QIO_CHANNEL(opaque);
> > > > 
> > > >     qio_channel_shutdown(ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
> > > > }
> > > > 
> > > > Where qio_channel_shutdown for tls is nothing but delivers that to the master
> > > > channel:
> > > > 
> > > > static int qio_channel_tls_shutdown(QIOChannel *ioc,
> > > >                                     QIOChannelShutdown how,
> > > >                                     Error **errp)
> > > > {
> > > >     QIOChannelTLS *tioc = QIO_CHANNEL_TLS(ioc);
> > > > 
> > > >     qatomic_or(&tioc->shutdown, how);
> > > > 
> > > >     return qio_channel_shutdown(tioc->master, how, errp);
> > > > }
> > > > 
> > > > So I thought it was a nice wrapper just for things like this, and I didn't see
> > > > anything it does more than the io_shutdown for the socket channel.  Did I miss
> > > > something?
> > > 
> > > Today thats the case, but don't assume it will be the case forever.
> > > There is a mechanism in TLS for doing clean shutdown which we've
> > > debated including.
> > > 
> > > In general apps *can* just call the shutdown method on the QIOChannelTLS
> > > object no matter what.  Yank is just a little bit special because of its
> > > need to be guaranteed to work even when the network is dead. So yank
> > > should always directly call the low level QIOChannelSocket, so thre is
> > > a strong guarantee it can't block on something.
> > 
> > Hmm, I am still not fully convinced that that's a valid reason the migration
> > code should be aware of how the socket is managed in tls channels...
> > 
> > Does that sound more like a good reason to introduce QIOChannelShutdown with
> > QIO_CHANNEL_SHUTDOWN_FORCE so it'll always not block if FORCE set?  Then we can
> > switch the yank function to use that.
> > 
> > What do you think?
> 
> I think that's unneccessary - the migration code already does similar
> things elsewhere when it wants to distinguish TLS usage, so this is not
> anything new conceptually.

I'd probably agree with Peter here that it would be preferential for the
migration code not to know;  what we want to do here is give the channel
the opportunity to do a hard shutdown, and let the channel worry about
it whether it can do that.

If you look at the nbd code, it's yank registration calls
qio_channel_shutdown on whatever channel it happens to have; so that's
probably wrong since I think you're saying that could block.

Dave

> Regards,
> Daniel
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK