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Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors |
Date: |
Wed, 20 May 2015 11:44:19 -0300 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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?
--
Eduardo
- Re: [Qemu-devel] [PATCH v4 4/8] qom: add helper method for getting user objects root, (continued)
- [Qemu-devel] [PATCH v4 8/8] qom: don't pass string table to object_get_enum method, Daniel P. Berrange, 2015/05/13
- [Qemu-devel] [PATCH v4 2/8] doc: document user creatable object types in help text, Daniel P. Berrange, 2015/05/13
- [Qemu-devel] [PATCH v4 6/8] qom: make enum string tables const-correct, Daniel P. Berrange, 2015/05/13
- [Qemu-devel] [PATCH v4 7/8] qom: add a object_property_add_enum helper method, Daniel P. Berrange, 2015/05/13
- [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 <=
- 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, 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, 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