Re: [Qemu-devel] [RFC v2 13/22] monitor: separate QMP parser and dispatcher

[Stefan Hajnoczi]

On Thu, Oct 19, 2017 at 02:36:49PM +0800, Peter Xu wrote:
> On Wed, Oct 18, 2017 at 05:31:15PM +0200, Stefan Hajnoczi wrote:
> > On Mon, Oct 16, 2017 at 03:50:39PM +0800, Peter Xu wrote:
> > > On Thu, Oct 12, 2017 at 01:50:45PM +0100, Stefan Hajnoczi wrote:
> > > > On Fri, Sep 29, 2017 at 11:38:35AM +0800, Peter Xu wrote:
> > > > > +    qdict = qobject_to_qdict(req);
> > > > > +    if (qdict) {
> > > > > +        id = qdict_get(qdict, "id");
> > > > > +        qobject_incref(id);
> > > > > +        qdict_del(qdict, "id");
> > > > > +    } /* else will fail qmp_dispatch() */
> > > > > +
> > > > > +    req_obj = g_new0(QMPRequest, 1);
> > > > > +    req_obj->mon = mon;
> > > > > +    req_obj->id = id;
> > > > > +    req_obj->req = req;
> > > > > +
> > > > > +    /*
> > > > > +     * Put the request to the end of queue so that requests will be
> > > > > +     * handled in time order.  Ownership for req_obj, req, id,
> > > > > +     * etc. will be delivered to the handler side.
> > > > > +     */
> > > > > +    g_queue_push_tail(mon->qmp_requests, req_obj);
> > > > > +
> > > > > +    /* Kick the dispatcher routine */
> > > > > +    qemu_bh_schedule(mon_global.qmp_dispatcher_bh);
> > > > 
> > > > How is thread-safety ensured when accessing qmp_requests?
> > > 
> > > It's a GQueue.  I assume GQueue is thread safe itself as long as
> > > g_thread_init() is called?
> > 
> > No, glib data structures are not automatically thread-safe unless the
> > documentation says so.
> > 
> > Here is the implementation where you can see that no locking is
> > performed:
> > 
> >   void
> >   g_queue_push_tail (GQueue   *queue,
> >                      gpointer  data)
> >   {
> >     g_return_if_fail (queue != NULL);
> > 
> >     queue->tail = g_list_append (queue->tail, data);
> >     if (queue->tail->next)
> >       queue->tail = queue->tail->next;
> >     else
> >       queue->head = queue->tail;
> >     queue->length++;
> >   }
> 
> Oops. Yes then I possibly need a lock...  Thanks.
> 
> I think maybe I can rename the Monitor.out_lock into a more general
> name, then to use it as a per-monitor lock (instead of introducing
> another one).

Up to you.  I don't remember the details of out_lock usage well enough
to know whether using the lock for the queues is good or bad.

Stefan