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 <pet...@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 <leobra...@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.
Regards,
Daniel
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