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

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Message-Id: <8922f6f0-241a-4659-b382-fb8c62b77e8f@app.fastmail.com>
Date: Thu, 26 Jun 2025 16:45:15 -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 4:36 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?

Regards,
Boqun

> ---
> Cheers,
> Benno