Re: [Qemu-devel] [PATCH v4 for-4.0 1/7] char-socket: Enable "nowait" option on client sockets

[Daniel P . Berrangé]

On Tue, Jan 15, 2019 at 07:53:51PM +0300, Yury Kotov wrote:
> 15.01.2019, 18:39, "Daniel P. Berrangé" <address@hidden>:
> > On Fri, Jan 11, 2019 at 04:36:11PM +0800, Yongji Xie wrote:
> >>  On Fri, 11 Jan 2019 at 16:32, Daniel P. Berrangé <address@hidden> wrote:
> >>  >
> >>  > On Fri, Jan 11, 2019 at 03:50:40PM +0800, Yongji Xie wrote:
> >>  > > On Fri, 11 Jan 2019 at 00:41, Daniel P. Berrangé <address@hidden> 
> >> wrote:
> >>  > > >
> >>  > > > We need to fix qemu_chr_fe_wait_connected so that it does explicit
> >>  > > > synchronization wrt to any ongoing background connection process.
> >>  > > > It must only return once all TLS/telnet/websock handshakes have
> >>  > > > completed. If we fix that correctly, then I believe it will also
> >>  > > > solve the problem you're trying to address.
> >>  > > >
> >>  > >
> >>  > > Yes, I think this should be the right way to go. To fix it, my thought
> >>  > > is to track the async QIOChannelSocket in SocketChardev. Then we can
> >>  > > easily get the connection progress in qemu_chr_fe_wait_connected(). Do
> >>  > > you have any suggestion?
> >>  >
> >>  > I've got a few patches that refactor the code to fix this. I'll send 
> >> them
> >>  > today and CC you on them.
> >>  >
> >>
> >>  That would be great! Thank you.
> >
> > It took me rather longer than expected to fully debug all scenarios, but
> > I've finally sent patches:
> >
> > https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg03344.html
> 
> I didn't do a deep review and may be wrong, but I think the race is still
> possible in the hotplug case.
> 
> Example:
> 1. User adds a chardev (qmp: chardev-add) with 'reconnect' option;
> 2. Some main-loop iterations...
> 3. Reconnect timer triggers and starts connection thread;
> 4. User adds a vhost-user-blk (qmp: device-add): device realize -> 
> wait_connected.
> 
> Here, there is a chance that we are in the wait_connected and the connection
> thread is still running.

Hmm, dealing with device-add is tricky. tcp_chr_wait_connect rather
assumes no main loop is running. I'll have to think about how we could
possibly handle hotplug safely.

Regards,
Daniel
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