linux-kernel - Re: [PATCH v10 07/13] rust: hrtimer: implement `UnsafeHrTimerPointer` for `Pin<&T>`

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Message-ID: <D8A2ZPH3TSGK.IZ41SSRCL37@proton.me>
Date: Fri, 07 Mar 2025 13:51:01 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Danilo Krummrich <dakr@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Lyude Paul <lyude@...hat.com>, Guangbo Cui <2407018371@...com>, Dirk Behme <dirk.behme@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, Tamir Duberstein <tamird@...il.com>, Markus Elfring <Markus.Elfring@....de>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 07/13] rust: hrtimer: implement `UnsafeHrTimerPointer` for `Pin<&T>`

On Fri Mar 7, 2025 at 2:37 PM CET, Andreas Hindborg wrote:
> "Benno Lossin" <benno.lossin@...ton.me> writes:
>> On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
>>> +impl<'a, T> RawHrTimerCallback for Pin<&'a T>
>>> +where
>>> +    T: HasHrTimer<T>,
>>> +    T: HrTimerCallback<Pointer<'a> = Self>,
>>> +{
>>> +    type CallbackTarget<'b> = Self;
>>> +
>>> +    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
>>> +        // `HrTimer` is `repr(C)`
>>> +        let timer_ptr = ptr as *mut HrTimer<T>;
>>> +
>>> +        // SAFETY: By the safety requirement of this function, `timer_ptr`
>>> +        // points to a `HrTimer<T>` contained in an `T`.
>>> +        let receiver_ptr = unsafe { T::timer_container_of(timer_ptr) };
>>> +
>>> +        // SAFETY: By the safety requirement of this function, `timer_ptr`
>>> +        // points to a `HrTimer<T>` contained in an `T`.
>>
>> This justification seems wrong it talks about `HrTimer<T>`, but here we
>> have a `*const T`... Also see [1] (I am mainly interested in your
>> justification for the lifetime).
>>
>> [1]: https://doc.rust-lang.org/std/ptr/index.html#pointer-to-reference-conversion
>
> How is this:
>
>         // SAFETY:
>         //  - By the safety requirement of this function, `timer_ptr`
>         //    points to a `HrTimer<T>` contained in an `T`.
>         //  - The `PinHrTimerHandle` associated with this timer is guaranteed to
>         //    be alive until this method returns. As the handle borrows from
>         //    `T`, `T` is also guaranteed to be alive for the duration of this
>         //    function.

Sounds good, if you can also explain (probably somewhere else, as every
`RawHrTimerCallback` implementer will rely on this) why the handle lives
for the duration of the callback.

---
Cheers,
Benno