Re: [Qemu-devel] [RFC v5 16/26] monitor: separate QMP parser and dispatcher

[Stefan Hajnoczi]

On Sat, Dec 16, 2017 at 02:37:03PM +0800, Peter Xu wrote:
> On Wed, Dec 13, 2017 at 08:09:38PM +0000, Stefan Hajnoczi wrote:
> > On Tue, Dec 05, 2017 at 01:51:50PM +0800, Peter Xu wrote:
> > > @@ -3956,12 +3968,122 @@ static void handle_qmp_command(JSONMessageParser 
> > > *parser, GQueue *tokens,
> > >          }
> > >      }
> > >  
> > > -err_out:
> > > -    monitor_qmp_respond(mon, rsp, err, id);
> > > +    /* Respond if necessary */
> > > +    monitor_qmp_respond(mon, rsp, NULL, id);
> > > +
> > > +    /* This pairs with the monitor_suspend() in handle_qmp_command(). */
> > > +    if (!qmp_oob_enabled(mon)) {
> > > +        monitor_resume(mon);
> > 
> > monitor_resume() does not work between threads: if the event loop is
> > currently blocked in poll() it won't notice that the monitor fd should
> > be watched again.
> > 
> > Please add aio_notify() to monitor_resume() and monitor_suspend().  That
> > way the event loop is forced to check can_read() again.
> 
> Ah, yes.  I think monitor_suspend() does not need the notify?  Since
> if it's sleeping it won't miss the next check in can_read() after all?

No, that would be a bug.  Imagine the IOThread is blocked in poll
monitoring the chardev file descriptor when the main loop calls
monitor_suspend().  If the file descriptors becomes readable then the
handler function executes even though the monitor is supposed to be
suspended!

> Regarding to monitor_resume(), I noticed that I missed the fact that
> it's only tailored for HMP before, which seems to be a bigger problem.
> Do you agree with a change like this?
> 
> diff --git a/monitor.c b/monitor.c
> index 9970418d6f..8f96880ad7 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -4244,10 +4244,12 @@ int monitor_suspend(Monitor *mon)
>  
>  void monitor_resume(Monitor *mon)
>  {
> -    if (!mon->rs)
> -        return;
>      if (atomic_dec_fetch(&mon->suspend_cnt) == 0) {
> -        readline_show_prompt(mon->rs);
> +        if (monitor_is_qmp(mon)) {
> +            aio_notify(mon_global.mon_iothread->ctx);

Please use iothread_get_aio_context() instead of accessing struct
members.

> +        } else {
> +            assert(mon->rs);
> +            readline_show_prompt(mon->rs);

I haven't studied the HMP and ->rs code.  I'll do that when reviewing
the next revision of this series.

> +        }
>      }
>  }
> 
> Even, I'm thinking about whether I should start to introduce
> iothread_notify() now to mask out the IOThread.ctx details.

aio_notify() is a low-level AioContext operation that is somewhat
bug-prone.  I think it's better to leave it directly exposed so callers
have to think about what they are doing.

Stefan

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