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

[Peter Xu <peterx@redhat.com>]

On Mon, Mar 26, 2018 at 11:46:13AM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Mon, Mar 26, 2018 at 11:08 AM, Peter Xu <peterx@redhat.com> wrote:
> > On Mon, Mar 26, 2018 at 10:33:27AM +0200, Marc-André Lureau wrote:
> >> Hi
> >>
> >> On Mon, Mar 26, 2018 at 10:07 AM, Peter Xu <peterx@redhat.com> wrote:
> >> > 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.
> >>
> >> But monitor_qmp_dispatch_one() is called from iothread if the command
> >> is oob, so cur_mon may be updated while another command is running in
> >> main thread, or am I wrong?
> >
> > You are right. I missed that, sorry...
> >
> > Would this be a simple workaround (but hopefully efficient) solution?
> >
> > diff --git a/monitor.c b/monitor.c
> > index 77f4c41cfa..99641c0c6d 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -4023,7 +4023,7 @@ typedef struct QMPRequest QMPRequest;
> >   * 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)
> > +static void monitor_qmp_dispatch_one(QMPRequest *req_obj, bool hack_curmon)
> >  {
> >      Monitor *mon, *old_mon;
> >      QObject *req, *rsp = NULL, *id;
> > @@ -4043,12 +4043,16 @@ static void monitor_qmp_dispatch_one(QMPRequest *req_obj)
> >          QDECREF(req_json);
> >      }
> >
> > -    old_mon = cur_mon;
> > -    cur_mon = mon;
> > +    if (hack_curmon) {
> > +        old_mon = cur_mon;
> > +        cur_mon = mon;
> > +    }
> >
> >      rsp = qmp_dispatch(mon->qmp.commands, req);
> >
> > -    cur_mon = old_mon;
> > +    if (hack_curmon) {
> > +        cur_mon = old_mon;
> > +    }
> >
> >      if (mon->qmp.commands == &qmp_cap_negotiation_commands) {
> >          qdict = qdict_get_qdict(qobject_to(QDict, rsp), "error");
> > @@ -4116,7 +4120,7 @@ static void monitor_qmp_bh_dispatcher(void *data)
> >
> >      if (req_obj) {
> >          trace_monitor_qmp_cmd_in_band(qobject_get_try_str(req_obj->id) ?: "");
> > -        monitor_qmp_dispatch_one(req_obj);
> > +        monitor_qmp_dispatch_one(req_obj, true);
> >          /* Reschedule instead of looping so the main loop stays responsive */
> >          qemu_bh_schedule(mon_global.qmp_dispatcher_bh);
> >      }
> > @@ -4175,7 +4179,7 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
> >          /* Out-Of-Band (OOB) requests are executed directly in parser. */
> >          trace_monitor_qmp_cmd_out_of_band(qobject_get_try_str(req_obj->id)
> >                                            ?: "");
> > -        monitor_qmp_dispatch_one(req_obj);
> > +        monitor_qmp_dispatch_one(req_obj, false);
> >          return;
> >      }
> >
> > Then we forbit touching that evil cur_mon in OOB-capable command
> > handlers.  Thanks,
> 
> That's not easy to enforce though, afaict it is being used for:
> - error reporting decision

IMO this should not be a problem, since any QMP handler (including
OOB-capable ones) will be with an Error** there, so logically speaking
people should never call things like error_report() in that.

> - file & socket lookup (fd: & /dev/fdset etc)

I suppose only very rare commands will use it?  It'll be a big problem
to solve when we want to completely remove cur_mon though.

> - the current state of the monitor / list of commands, cpu_path, capabilities..

This is very rare to be used too?  Most commands should not use them AFAIU.

> 
> Wouldn't it be simpler to make it per-thread? I think it could also
> use helpers to push/pop the current monitor.

Anyway I think yes this is still a good option (though the cur_mon
logic will be a bit more complicated).

Do you plan to post some patch about this, or do you want me to do
this?  I suppose we'll change the qemu_thread_create() a bit to pass
the cur_mon inside, and I suppose this might be better material after
2.12 release if OOB is off now.

-- 
Peter Xu