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

[Anthony Liguori <anthony@codemonkey.ws>]

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<pbonzini@redhat.com>
>>
>> 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
>
>