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

[Andreas Hindborg <a.hindborg@kernel.org>]

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

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