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

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Message-Id: <DAWUY4YH6XP9.TWAP6N95L5BR@kernel.org>
Date: Fri, 27 Jun 2025 01:36:01 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>, "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 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.

---
Cheers,
Benno