Re: [Qemu-devel] [PATCH 01/16] qdev: fix hot-unplug

[Anthony Liguori]

On 02/03/2012 06:27 PM, Paolo Bonzini wrote:
> On 02/03/2012 03:27 PM, Anthony Liguori wrote:
> > On 02/02/2012 10:45 AM, Paolo Bonzini wrote:
> > > The reference that is returned by qdev_device_add is never given
> > > back, so that device_del does not cause the refcount to go to zero
> > > (and thus does nothing).
> > >
> > > Signed-off-by: Paolo Bonzini<address@hidden>
> >
> > This isn't needed in qom-upstream.14. Here's why:
> >
> > object_init does not increase the reference count
> >
> > object_property_add_child increases the reference count
> > object_new increases the reference count
> >
> > object_delete decrements the reference count
> > object_property_del_child decreases the reference count
> >
> > object_delete calls object_property_del_child(obj->parent, obj)
> >
> > qdev_device_add calls object_new and object_property_add_child
> > -> ref == 2
> >
> > qdev_device_del calls object_delete
> > -> ref -= 2
> >
> > In qom-upstream.13, object_delete wasn't calling
> > object_property_del_child which is why you saw the behavior you did.
> > This problem would still exist with a composed device so dropping the
> > reference here isn't enough.
>
> I trust you for now. :)
>
> It really seems like my patch is obviously correct so, if it's not needed
> anymore there may be another bug elsewhere that masks it.

There's no object_ref() in qdev_device_add().  The 2 references come from adding 
a child link to /peripheral and via object_new().

object_free() drops a reference (it's called in qdev_device_del()) and in the 
process of calling object_free(), it also calls object_unparent() which will 
drop the reference from the parent.

I'm not thrilled about the way reference counting is done now.  Perhaps we 
should do a gobject style floating reference...

Regards,

Anthony Liguori

> Paolo