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Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_
From: |
Andreas Färber |
Subject: |
Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors |
Date: |
Wed, 20 May 2015 18:11:33 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 |
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
--
SUSE Linux GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Felix Imendörffer, Jane Smithard, Dilip Upmanyu, Graham Norton; HRB
21284 (AG Nürnberg)
- [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, (continued)
- [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Daniel P. Berrange, 2015/05/13
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Andreas Färber, 2015/05/19
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Daniel P. Berrange, 2015/05/19
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Andreas Färber, 2015/05/19
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Paolo Bonzini, 2015/05/19
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Eduardo Habkost, 2015/05/20
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Daniel P. Berrange, 2015/05/20
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Andreas Färber, 2015/05/20
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Eduardo Habkost, 2015/05/20
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Daniel P. Berrange, 2015/05/20
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors,
Andreas Färber <=
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Eduardo Habkost, 2015/05/20
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Andreas Färber, 2015/05/20
- Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors, Eduardo Habkost, 2015/05/20