Re: [PATCH v10 03/13] rust: hrtimer: implement `HrTimerPointer` for `Arc`

[Benno Lossin <benno.lossin@proton.me>]

On Fri Mar 7, 2025 at 2:27 PM CET, Andreas Hindborg wrote:
> "Benno Lossin" <benno.lossin@proton.me> writes:
>
>> On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
>>> +impl<T> HrTimerPointer for Arc<T>
>>> +where
>>> +    T: 'static,
>>> +    T: Send + Sync,
>>> +    T: HasHrTimer<T>,
>>> +    T: for<'a> HrTimerCallback<Pointer<'a> = Self>,
>>> +    Arc<T>: for<'a> RawHrTimerCallback<CallbackTarget<'a> = ArcBorrow<'a, T>>,
>>
>> I don't understand why you need this bound here.
>
> This impl is applicable only when `Arc<T> has an implementation of
> `RawTimerCallback` where CallbackTarget<'a> = ArcBorrow<'a, T>. I don't
> want the impl to be available if that is not the case.

The impl below has less strict other bounds than this one, so this bound
doesn't change anything.

> It's just an additional check.

To me it's just additional noise.

>>> +{
>>> +    type TimerHandle = ArcHrTimerHandle<T>;
>>> +
>>> +    fn start(self, expires: Ktime) -> ArcHrTimerHandle<T> {
>>> +        // SAFETY:
>>> +        //  - We keep `self` alive by wrapping it in a handle below.
>>> +        //  - Since we generate the pointer passed to `start` from a valid
>>> +        //    reference, it is a valid pointer.
>>> +        unsafe { T::start(Arc::as_ptr(&self), expires) };
>>> +        ArcHrTimerHandle { inner: self }
>>> +    }
>>> +}
>>> +
>>> +impl<T> RawHrTimerCallback for Arc<T>
>>> +where
>>> +    T: 'static,
>>> +    T: HasHrTimer<T>,
>>> +    T: for<'a> HrTimerCallback<Pointer<'a> = Self>,
>>> +{
>>> +    type CallbackTarget<'a> = ArcBorrow<'a, T>;
>>> +
>>> +    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
>>> +        // `HrTimer` is `repr(C)`
>>> +        let timer_ptr = ptr.cast::<super::HrTimer<T>>();
>>> +
>>> +        // SAFETY: By C API contract `ptr` is the pointer we passed when
>>> +        // queuing the timer, so it is a `HrTimer<T>` embedded in a `T`.
>>> +        let data_ptr = unsafe { T::timer_container_of(timer_ptr) };
>>> +
>>> +        // SAFETY: `data_ptr` points to the `T` that was used to queue the
>>> +        // timer. This `T` is contained in an `Arc`.
>>
>> You're not justifying all safety requirements of `ArcBorrow::from_raw`.
>
> How is this:
>
>         // SAFETY:
>         //  - `data_ptr` is derived form the pointer to the `T` that was used to
>         //    queue the timer.
>         //  - The `ArcTimerHandle` associated with this timer is guaranteed to
>         //    be alive for the duration of the lifetime of `receiver`, so the

There is no `receiver` in this context?

Is the reason for the handle staying alive that when it is dropped, it
calls `cancel` and that waits until the callback finishes? If so, did
you write that down somewhere here?

>         //    refcount of the underlying `Arc` is guaranteed to be nonzero for
>         //    the duration.
>         //  - We own one refcount in the `ArcTimerHandle` associted with this
>         //    timer, so it is not possible to get a `UniqueArc` to this
>         //    allocation from other `Arc` clones.

Otherwise this sounds good.

---
Cheers,
Benno