Re: [Qemu-devel] [PATCH] monitor: postpone monitor_qmp_cleanup_queues

[Stefan Hajnoczi]

On Tue, Jun 12, 2018 at 01:47:00PM +0800, Peter Xu wrote:
> On Mon, Jun 11, 2018 at 05:45:49PM +0100, Stefan Hajnoczi wrote:
> > On Fri, Jun 08, 2018 at 10:18:25AM +0200, Markus Armbruster wrote:
> > > Stefan Hajnoczi <address@hidden> writes:
> > > > On Fri, Jun 08, 2018 at 12:42:35PM +0800, Peter Xu wrote:
> > > >> On Thu, Jun 07, 2018 at 01:53:01PM +0200, Markus Armbruster wrote:
> > > >> > Peter Xu <address@hidden> writes:
> > > >> > 
> > > >> > > Previously we cleanup the queues when we got CLOSED event.  It was 
> > > >> > > used
> > > >> > 
> > > >> > we clean up
> > > >> > 
> > > >> > > to make sure we won't leftover replies/events of a old client to a 
> > > >> > > new
> > > >> > 
> > > >> > we won't send leftover replies/events of an old client
> > > >> > 
> > > >> > > client.  Now this patch postpones that until OPENED.
> > > >> > 
> > > >> > What if OPENED never comes?
> > > >> 
> > > >> Then we clean that up until destruction of the monitor.  IMHO it's
> > > >> fine, but I'm not sure whether that's an overall acceptable approach.
> > > >
> > > > I think this patch fixes the problem at the wrong level.  Marc-André's
> > > > fix seemed like a cleaner solution.
> > > 
> > > Is it the right solution?
> > > 
> > > I proposed another one:
> > 
> > Sorry, I won't be able to participate in this because I'm behind on
> > other patches and tasks.  Therefore, feel free to disregard but I'll
> > give my 2 cents:
> > 
> > This seems like a chardev bug.  The solution should probably be in the
> > chardev layer (Marc-André's patch or something else), not in the monitor
> > (this patch).
> 
> Yes that's why I said I like Marc-Andre's patch. :) I just don't know
> an reliable way to achieve what we want there.
> 
> The thing is that we don't really monitor the ioc_out for fd-typed
> chardevs.  We only do that when we call qemu_chr_fe_add_watch() (e.g.,
> in monitor_flush_locked()) when the writting buffer is full.  But
> normally we can't detect any event from the output side, hence no way
> to deliever a CLOSED event when the output fd is the last fd that is
> closed.
> 
> Maybe we can keep that output watch for the whole lifecycle of the
> chardev?  I'm not sure yet.

It's not possible to keep a POLLOUT fd watch since poll(2) would return
instantly and the event loop would spin.  However, POLLHUP (G_IO_HUP)
might work.

Stefan

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