From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46779) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Yv5l7-0005aO-B2 for qemu-devel@nongnu.org; Wed, 20 May 2015 11:18:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Yv5l2-00037O-Ay for qemu-devel@nongnu.org; Wed, 20 May 2015 11:18:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53890) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Yv5l2-00037F-4X for qemu-devel@nongnu.org; Wed, 20 May 2015 11:18:08 -0400 Date: Wed, 20 May 2015 16:18:03 +0100 From: "Daniel P. Berrange" Message-ID: <20150520151803.GA23989@redhat.com> References: <1431533649-23115-1-git-send-email-berrange@redhat.com> <1431533649-23115-6-git-send-email-berrange@redhat.com> <555B5C2E.5050903@suse.de> <20150519155528.GF8535@redhat.com> <555B6099.4060803@redhat.com> <20150520144419.GE28075@thinpad.lan.raisama.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20150520144419.GE28075@thinpad.lan.raisama.net> Subject: Re: [Qemu-devel] [PATCH v4 5/8] qom: add object_new_with_props / object_new_withpropv constructors Reply-To: "Daniel P. Berrange" List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: Paolo Bonzini , Andreas =?utf-8?Q?F=C3=A4rber?= , qemu-devel@nongnu.org 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. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|