Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, May 20, 2015 at 11:44:19AM -0300, Eduardo Habkost wrote:
> On Tue, May 19, 2015 at 06:11:05PM +0200, Paolo Bonzini wrote:
> > On 19/05/2015 17:55, Daniel P. Berrange wrote:
> > > Paolo told me on previous posting that object_property_add_child()
> > > holds a reference on 'obj' for as long as it is registered in the
> > > object hierarchy composition. So it sufficient to rely on that long
> > > term reference, and let the caller dispose of the object by calling
> > > object_unparent(obj) when finally done.
> > 
> > For an example of the same pattern:
> > 
> > DeviceState *qdev_try_create(BusState *bus, const char *type)
> > {
> >     DeviceState *dev;
> > 
> >     if (object_class_by_name(type) == NULL) {
> >         return NULL;
> >     }
> >     dev = DEVICE(object_new(type));
> >     if (!dev) {
> >         return NULL;
> >     }
> > 
> >     if (!bus) {
> >         bus = sysbus_get_default();
> >     }
> > 
> >     qdev_set_parent_bus(dev, bus);
> >     object_unref(OBJECT(dev));
> >     return dev;
> > }
> > 
> > Effectively this is idea as GObject's "floating reference".
> > qdev_set_parent_bus (in qdev_try_create) and object_property_add_child
> > (in Daniel's patches) "sink" the floating reference by doing
> > object_unref.  If we had floating references, the object would be
> > returned to the caller unref'ed anyway.
> 
> I was agreeing with Andreas at first (because it would make the
> reference ownership rules simpler and easier to understand), until I
> noticed that every call of qdev_try_create() and object_resolve_path()
> in the code would need an additional object_unref() call if we didn't
> use this pattern.
> 
> But it bothers me that this exceptional behavior is not documented on
> neither qdev_try_create() or object_resolve_path().
> 
> > 
> > Of course, the reference can go away via QMP.  But that will only happen
> > after the caller would have called object_unref itself.
> 
> But the caller won't ever call object_unref() because it doesn't own any
> reference, right? In this case, can we clarify the rules about how long
> can callers safely expect the object to stay around? Can the object be
> disposed in another thread? Can it be disposed only when some specific
> events happen?

In the inline docs for object_new_with_props I wrote

  * The returned object will have one stable reference maintained
  * for as long as it is present in the object hierarchy.

We could expand it to explicitly say that 'object_unparent' is required
to remove the object from the hierarchy and free it.

Regards,
Daniel
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