linux-kernel - Re: [PATCH v4 4/5] rust: types: ForeignOwnable: Add type Target

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Message-Id: <44579f29-a8a4-41cb-97ea-5ab7711e4d2a@app.fastmail.com>
Date: Thu, 26 Jun 2025 16:55:50 -0700
From: "Boqun Feng" <boqun.feng@...il.com>
To: "Benno Lossin" <lossin@...nel.org>, "Danilo Krummrich" <dakr@...nel.org>
Cc: "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>, rafael@...nel.org,
 "Miguel Ojeda" <ojeda@...nel.org>, alex.gaynor@...il.com,
 "Gary Guo" <gary@...yguo.net>, bjorn3_gh@...tonmail.com,
 "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <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 4/5] rust: types: ForeignOwnable: Add type Target



On Thu, Jun 26, 2025, at 4:45 PM, Boqun Feng wrote:
> On Thu, Jun 26, 2025, at 4:36 PM, Benno Lossin wrote:
>> On Fri Jun 27, 2025 at 1:21 AM CEST, Boqun Feng wrote:
>>> On Thu, Jun 26, 2025, at 4:17 PM, Benno Lossin wrote:
>>>> On Thu Jun 26, 2025 at 10:20 PM CEST, Boqun Feng wrote:
>>>>> On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
>>>>>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>>>>>> index 3958a5f44d56..74c787b352a9 100644
>>>>>> --- a/rust/kernel/types.rs
>>>>>> +++ b/rust/kernel/types.rs
>>>>>> @@ -27,6 +27,9 @@
>>>>>>  /// [`into_foreign`]: Self::into_foreign
>>>>>>  /// [`PointedTo`]: Self::PointedTo
>>>>>>  pub unsafe trait ForeignOwnable: Sized {
>>>>>> +    /// The payload type of the foreign-owned value.
>>>>>> +    type Target;
>>>>>
>>>>> I think `ForeignOwnable` also implies a `T` maybe get dropped via a
>>>>> pointer from `into_foreign()`. Not sure it's worth mentioning though.
>>>>
>>>> What? How would that happen?
>>>
>>> The owner of the pointer can do from_foreign() and eventually drop
>>> the ForeignOwnable, hence dropping T.
>>
>> I'm confused, you said `into_foreign` above. I don't think any sensible
>> ForeignOwnable implementation will drop a T in any of its functions.
>>
>
> A KBox<T> would drop T when it gets dropped, no?
> A Arc<T> would drop T when it gets dropped if it’s the last refcount.
>
> I was trying to say “ForeignOwnable::drop() may implies Target::drop()”,
> that’s what a “payload” means. Maybe that I used “T” instead of “Target”
> in the original message caused confusion?
>

The point is whichever receives the pointer from a into_foreign()
would owns the Target, because it can from_foreign() and
drop the ForeignOwnable. So for example, if the pointer can
be passed across threads, that means Target needs to be Send.

Regards,
Boqun

> Regards,
> Boqun
>
>> ---
>> Cheers,
>> Benno