Re: [Qemu-devel] [PATCH v2 0/5] Fix socket chardev regression

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 24, 2018 at 10:32:58AM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Fri, Aug 24, 2018 at 5:45 AM, Peter Xu <peterx@redhat.com> wrote:
> > On Thu, Aug 23, 2018 at 04:31:20PM +0200, Marc-André Lureau wrote:
> >> Hi,
> >>
> >> In commit 25679e5d58e "chardev: tcp: postpone async connection setup"
> >> (and its follow up 99f2f54174a59), Peter moved chardev socket
> >> connection to machine_done event. However, chardev created later will
> >> no longer attempt to connect, and chardev created in tests do not have
> >> machine_done event (breaking some of vhost-user-test).
> >>
> >> The goal was to move the "connect" source to the chardev frontend
> >> context (the monitor thread context in his case). chr->gcontext is set
> >> with qemu_chr_fe_set_handlers(). But there is no guarantee that the
> >> function will be called in general, so we can't delay connection until
> >> then: the chardev should still attempt to connect during open(), using
> >> the main context.
> >>
> >> An alternative would be to specify the iothread during chardev
> >> creation. Setting up monitor OOB would be quite different too, it
> >> would take the same iothread as argument.
> >>
> >> 99f2f54174a595e is also a bit problematic, since it will behave
> >> differently before and after machine_done (the first case gives a
> >> chance to use a different context reliably, the second looks racy)
> >>
> >> In the end, I am not sure this is all necessary, as chardev callbacks
> >> are called after qemu_chr_fe_set_handlers(), at which point the
> >> context of sources are updated. In "char-socket: update all ioc
> >> handlers when changing context", I moved also the hup handler to the
> >> updated context. So unless the main thread is already stuck, we can
> >> setup a different context for the chardev at that time. Or not?
> >>
> >> v2:
> >> - fix a random socket chardev test failure
> >
> > I have no problem on patch 3-5, but aren't patch 1-2 still breaking
> > the context switch of chardev?  Or did I misunderstood?
> 
> Switching context is broken/racy regardless. My current plan is to
> suspend the context to switch to, and update all sources before
> thawing the target context.
> 
> If you prefer we can delay the revert, but I would rather have them
> because the regression seems much worse to me.

Have you started working on above?  I started to code up some RFC
patch to let the chardev maintain the iothreads (now it'll only have a
monitor iothread) then we don't need to switch context for chardev any
more.  I plan to post it today for a quick look.  With that, this
series will not break anything then.

> 
> >
> > Regards,
> >
> > --
> > Peter Xu

Regards,

-- 
Peter Xu