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

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

"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.

It's just an additional check.

>
>> +{
>> +    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
        //    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.


Best regards,
Andreas Hindborg