linux-kernel - Re: [PATCH v4 5/5] rust: devres: implement register

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Message-Id: <DAXXVXNTRLYH.1B8O2LKBF4EW1@kernel.org>
Date: Sat, 28 Jun 2025 08:06:52 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: "Danilo Krummrich" <dakr@...nel.org>, <gregkh@...uxfoundation.org>,
 <rafael@...nel.org>, <ojeda@...nel.org>, <alex.gaynor@...il.com>,
 <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>, <a.hindborg@...nel.org>,
 <aliceryhl@...gle.com>, <tmgross@...ch.edu>, <david.m.ertman@...el.com>,
 <ira.weiny@...el.com>, <leon@...nel.org>, <kwilczynski@...nel.org>,
 <bhelgaas@...gle.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v4 5/5] rust: devres: implement register_release()

On Sat Jun 28, 2025 at 12:06 AM CEST, Boqun Feng wrote:
> On Fri, Jun 27, 2025 at 01:19:53AM +0200, Benno Lossin wrote:
>> On Thu Jun 26, 2025 at 10:48 PM CEST, Danilo Krummrich wrote:
>> > On Thu, Jun 26, 2025 at 01:37:22PM -0700, Boqun Feng wrote:
>> >> On Thu, Jun 26, 2025 at 10:00:43PM +0200, Danilo Krummrich wrote:
>> >> > +/// [`Devres`]-releaseable resource.
>> >> > +///
>> >> > +/// Register an object implementing this trait with [`register_release`]. Its `release`
>> >> > +/// function will be called once the device is being unbound.
>> >> > +pub trait Release {
>> >> > +    /// The [`ForeignOwnable`] pointer type consumed by [`register_release`].
>> >> > +    type Ptr: ForeignOwnable;
>> >> > +
>> >> > +    /// Called once the [`Device`] given to [`register_release`] is unbound.
>> >> > +    fn release(this: Self::Ptr);
>> >> > +}
>> >> > +
>> >> 
>> >> I would like to point out the limitation of this design, say you have a
>> >> `Foo` that can ipml `Release`, with this, I think you could only support
>> >> either `Arc<Foo>` or `KBox<Foo>`. You cannot support both as the input
>> >> for `register_release()`. Maybe we want:
>> >> 
>> >>     pub trait Release<Ptr: ForeignOwnable> {
>> >>         fn release(this: Ptr);
>> >>     }
>> >
>> > Good catch! I think this wasn't possible without ForeignOwnable::Target.
>> 
>> Hmm do we really need that? Normally you either store a type in a shared
>
> I think it might be quite common, for example, `Foo` may be a general
> watchdog for a subsystem, for one driver, there might be multiple
> devices that could feed the dog, for another driver, there might be only
> one. For the first case we need Arc<Watchdog> or the second we can do
> Box<Watchdog>.

I guess then the original `&self` design is better? Not sure...

> What's the downside?

You'll need to implement `Release` twice:

    impl Release<Box<Self>> for Foo {
        fn release(this: Box<Self>) {
            /* ... */
        }
    }

    impl Release<Arc<Self>> for Foo {
        fn release(this: Arc<Self>) {
            /* ... */
        }
    }

This also means that you can have different behavior for `Box` and
`Arc`...

---
Cheers,
Benno