linux-kernel - Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

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Message-ID: <87cyfbe89x.fsf@kernel.org>
Date: Fri, 21 Feb 2025 14:28:58 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Tamir Duberstein" <tamird@...il.com>
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>,  "Benno Lossin" <benno.lossin@...ton.me>,
  "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>,  <rust-for-linux@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

Andreas Hindborg <a.hindborg@...nel.org> writes:

> "Tamir Duberstein" <tamird@...il.com> writes:
>
>> On Thu, Feb 20, 2025 at 4:19 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>>
>>> "Tamir Duberstein" <tamird@...il.com> writes:
>>>
>>> > On Tue, Feb 18, 2025 at 8:29 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:

[...]

>>> >> +    /// Get a pointer to the contained `bindings::hrtimer`.
>>> >> +    ///
>>> >> +    /// # Safety
>>> >> +    ///
>>> >> +    /// `ptr` must point to a live allocation of at least the size of `Self`.
>>> >> +    unsafe fn raw_get(ptr: *const Self) -> *mut bindings::hrtimer {
>>> >> +        // SAFETY: The field projection to `timer` does not go out of bounds,
>>> >> +        // because the caller of this function promises that `ptr` points to an
>>> >> +        // allocation of at least the size of `Self`.
>>> >> +        unsafe { Opaque::raw_get(core::ptr::addr_of!((*ptr).timer)) }
>>> >> +    }
>>> >
>>> > Can you help me understand why the various functions here operate on
>>> > *const Self? I understand the need to obtain a C pointer to interact
>>> > with bindings, but I don't understand why we're dealing in raw
>>> > pointers to the abstraction rather than references.
>>>
>>> We cannot reference the `bindings::hrtimer` without wrapping it in
>>> `Opaque`. This would be the primary reason. At other times, we cannot
>>> produce references because we might not be able to prove that we satisfy
>>> the safety requirements for turning a pointer into a reference. If we
>>> are just doing offset arithmetic anyway, we don't need a reference.
>>
>> Why do we have a pointer, rather than a reference, to Self in the
>> first place? I think this is the key thing I don't understand.
>
> Perhaps it makes more sense if you look at the context. One of the entry
> points to `HrTimer::raw_get` is via `<ArcHrTimerHandle as
> HrTimerHandle>::cancel`. This user facing method takes `&mut self`. The
> handle contains an arc to a type that contains a `Timer` and implements
> `HasHrTImer`. To get to the timer, we need to do pointer manipulation.
> We only know how to get the `Timer` field via the `OFFSET`. The natural
> return value from the offset operation is a raw pointer. Rather than
> convert back to a reference, we stay in pointer land when we call
> `HrTimer::raw_cancel`, because we need a pointer to the
> `bindings::hrtimer` anyway, not a reference.

I changed `HasHrTimer::start` to take a reference, and I think that
makes sense 👍 Taking an `impl AsRef` does not work out when `Self` is
`Pin<&T>`. I'll go over the whole thing and see of other places could
benefit.

Best regards,
Andreas Hindborg