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

["Andreas Färber" <afaerber@suse.de>]

Am 20.05.2015 um 18:06 schrieb Eduardo Habkost:
> On Wed, May 20, 2015 at 04:18:03PM +0100, Daniel P. Berrange wrote:
>> 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.
> 
> What's missing to me is some clarification on how long it is safe to
> assume that the object won't be removed from the hierarchy by other
> code.

There is no guarantee. Therefore any caller of object_resolve_path()
must ref and unref as needed if doing more than a single operation, such
as setting a link target.

qdev is a different matter - there's too many instances to fix properly
right away. The current code serves to balance the ref count for
hot-unplug etc. to work. But it is not a template to copy for new QOM
code IMO.

Regards,
Andreas

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