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

[Stefan Hajnoczi]

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++;
  }

Stefan