Re: [Qemu-devel] [PATCH v8 14/23] monitor: separate QMP parser and dispatcher

[Peter Xu]

On Fri, Mar 23, 2018 at 05:18:53PM +0100, Marc-André Lureau wrote:

[...]

> > +/*
> > + * Dispatch one single QMP request. The function will free the req_obj
> > + * and objects inside it before return.
> > + */
> > +static void monitor_qmp_dispatch_one(QMPRequest *req_obj)
> >  {
> > -    QObject *req, *rsp = NULL, *id = NULL;
> > +    Monitor *mon, *old_mon;
> > +    QObject *req, *rsp = NULL, *id;
> >      QDict *qdict = NULL;
> > -    MonitorQMP *mon_qmp = container_of(parser, MonitorQMP, parser);
> > -    Monitor *old_mon, *mon = container_of(mon_qmp, Monitor, qmp);
> > -
> > -    Error *err = NULL;
> > +    bool need_resume;
> >
> > -    req = json_parser_parse_err(tokens, NULL, &err);
> > -    if (!req && !err) {
> > -        /* json_parser_parse_err() sucks: can fail without setting @err */
> > -        error_setg(&err, QERR_JSON_PARSING);
> > -    }
> > -    if (err) {
> > -        goto err_out;
> > -    }
> > +    req = req_obj->req;
> > +    mon = req_obj->mon;
> > +    id = req_obj->id;
> > +    need_resume = req_obj->need_resume;
> >
> > -    qdict = qobject_to_qdict(req);
> > -    if (qdict) {
> > -        id = qdict_get(qdict, "id");
> > -        qobject_incref(id);
> > -        qdict_del(qdict, "id");
> > -    } /* else will fail qmp_dispatch() */
> > +    g_free(req_obj);
> >
> >      if (trace_event_get_state_backends(TRACE_HANDLE_QMP_COMMAND)) {
> >          QString *req_json = qobject_to_json(req);
> > @@ -3900,7 +3932,7 @@ static void handle_qmp_command(JSONMessageParser 
> > *parser, GQueue *tokens)
> >      old_mon = cur_mon;
> >      cur_mon = mon;
> 
> There is another issue with this series, since cur_mon is global (and
> not protected), an oob command may change the cur_mon while another
> command is running in the main thread with unexpected consequences. I
> don't have a clear idea what is the best way to solve it. Making the
> variable per-thread, or going all the way to get rid of cur_mon (my
> preference, but much harder)

IMHO it is fine too.

Note that this cur_mon operation is in monitor_qmp_dispatch_one() now,
which is still running in main thread.  So AFAICT all the cur_mon
references are in main thread, and monitor IOThread does not modify
that variable at all.  Then we should probably be safe.

I would be far more than glad to see cur_mon go away one day.

Thanks,

-- 
Peter Xu