Re: [PATCH 02/10] rust: qom: add reference counting functionality

[Paolo Bonzini]

On 2/5/25 10:13, Zhao Liu wrote:
> > > * The use of from():
> > >
> > >                  let clk = bindings::qdev_init_clock_in(...)
> > >                  Owned::from(&*clk)
> >
> > In this case the C side wants to manage the reference that
> > qdev_init_clock_in() returns; it is dropped in
> > qdev_finalize_clocklist(). So Rust code needs to increase the
> > refcount.
>
> Pls forgive me for one more question about qdev_init_clock_in() on the C
> side. :-)
>
> qdev_init_clock_in() didn't unref `clk` after object_property_add_child(),
> so it is intentional, to make the ref count of `clk` be 2:
>   * 1 count is held by clocklist until qdev_finalize_clocklist().
>   * another 1 is held by its parent via QOM Child<>.
>
> Am I understanding it correctly?

Yes, that's more precise.  In Rust it will be 3, the two above plus the 
Owned<Clock>.

Ah wait... qdev_finalize_clocklist() is only called _after_ the Rust 
struct is Drop::drop-ped, because device_finalize() is called after the 
subclass's instance_finalize.

So the result of qdev_init_clock_in(), strictly speaking, does not have 
to be an Owned<Clock>.  It can also be a &'device Clock; either is 
possible.  Would you prefer that, or do you think it's enough to add a 
comment?

Paolo

> > > Then the comment "the clock is heap allocated and does not have
> > > a reference" sounds like a conflict. I'm sure I'm missing something. :-(
> >
> > Changed:
> >
> >        // SAFETY: the clock is heap allocated, but qdev_init_clock_in()
> >        // does not gift the reference to its caller; so use Owned::from to
> >        // add one.  the callback is disabled automatically when the clock
> >        // is unparented, which happens before the device is finalized.
>
> LGTM.
>
> Thank you very much for your patience. I think I understand ref count
> now.
>
> Regards,
> Zhao