Re: [PATCH v3 0/7] Do not unparent in instance_finalize()

[Akihiko Odaki <odaki@xxxxxxxxxxxxxxxxxxxxxx>]

On 2025/09/17 20:57, Daniel P. Berrangé wrote:
> On Wed, Sep 17, 2025 at 07:13:25PM +0900, Akihiko Odaki wrote:
> > Based-on: <cover.1751493467.git.balaton@xxxxxxxxxx>
> > ("[PATCH v2 00/14] hw/pci-host/raven clean ups")
> >
> > Supersedes: <20240829-memory-v1-1-ac07af2f4fa5@xxxxxxxxxx>
> > ("[PATCH] docs/devel: Prohibit calling object_unparent() for memory region")
> >
> > Children are automatically unparented so manually unparenting is
> > unnecessary.
> Where is automatic unparenting you're referring to being done ?
>
> > Worse, automatic unparenting happens before the instance_finalize()
> > callback of the parent gets called, so object_unparent() calls in
> > the callback will refer to objects that are already unparented, which
> > is semantically incorrect.
> IIUC, object_property_add_child will acquire a reference on
> the child, and object_property_del_child (and thus
> object_unparent) will release that reference.
>
> The 'object_finalize' method, and thus 'instance_finalize'
> callback, won't be invoked until the last reference is
> dropped on the object in question.
>
> IOW, it should be impossible for 'object_finalize' to ever
> run, as long as the child has a parent set.
>
> So if we're in the 'finalize' then 'object_unparent' must
> be a no-op as the child must already have no references
> held and thus no parent.
>
> IOW, the reason to remove 'object_unparent' calls from
> finalize is surely because they do nothing at all,
> rather than this talk about callbacks being run at the
> wrong time ?
This patch series deals with the situation where the parent calls 
object_unparent() in its instance_finalize() callback. The process of 
finalization looks like as follows:
1. The parent's reference count reaches to zero. Please note that there 
can be remaining children that are referenced by the parent at this point.
2. object_finalize() is called.

2a. object_property_del_all() is called and the parent releases 
references to its children. This is what I referred as "automatic 
unparenting". The children without any other references will be 
finalized here.
2b. instance_finalize() is called. Past children may be already 
finalized, and calling object_unparent() here will cause dereferencing 
finalized objects in that case, which should be avoided.
Regards,
Akihiko Odaki