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

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

On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
> Allow pinned references to structs that contain a `HrTimer` node to be
> scheduled with the `hrtimer` subsystem.
>
> Acked-by: Frederic Weisbecker <frederic@kernel.org>
> Reviewed-by: Lyude Paul <lyude@redhat.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/time/hrtimer.rs     |  2 +
>  rust/kernel/time/hrtimer/pin.rs | 99 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 101 insertions(+)
>
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index d90a25785f87..2ca56397eade 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -439,3 +439,5 @@ unsafe fn timer_container_of(ptr: *mut $crate::time::hrtimer::HrTimer<$timer_typ
>
>  mod arc;
>  pub use arc::ArcHrTimerHandle;
> +mod pin;
> +pub use pin::PinHrTimerHandle;
> diff --git a/rust/kernel/time/hrtimer/pin.rs b/rust/kernel/time/hrtimer/pin.rs
> new file mode 100644
> index 000000000000..6c9f2190f8e1
> --- /dev/null
> +++ b/rust/kernel/time/hrtimer/pin.rs
> @@ -0,0 +1,99 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use super::HasHrTimer;
> +use super::HrTimer;
> +use super::HrTimerCallback;
> +use super::HrTimerHandle;
> +use super::RawHrTimerCallback;
> +use super::UnsafeHrTimerPointer;
> +use crate::time::Ktime;
> +use core::pin::Pin;
> +
> +/// A handle for a `Pin<&HasHrTimer>`. When the handle exists, the timer might be
> +/// running.
> +pub struct PinHrTimerHandle<'a, T>
> +where
> +    T: HasHrTimer<T>,
> +{
> +    pub(crate) inner: Pin<&'a T>,
> +}
> +
> +// SAFETY: We cancel the timer when the handle is dropped. The implementation of
> +// the `cancel` method will block if the timer handler is running.
> +unsafe impl<'a, T> HrTimerHandle for PinHrTimerHandle<'a, T>
> +where
> +    T: HasHrTimer<T>,
> +{
> +    fn cancel(&mut self) -> bool {
> +        let self_ptr: *const T = self.inner.get_ref();
> +
> +        // SAFETY: As we got `self_ptr` from a reference above, it must point to
> +        // a valid `T`.
> +        let timer_ptr = unsafe { <T as HasHrTimer<T>>::raw_get_timer(self_ptr) };
> +
> +        // SAFETY: As `timer_ptr` is derived from a reference, it must point to
> +        // a valid and initialized `HrTimer`.
> +        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
> +    }
> +}
> +
> +impl<'a, T> Drop for PinHrTimerHandle<'a, T>
> +where
> +    T: HasHrTimer<T>,
> +{
> +    fn drop(&mut self) {
> +        self.cancel();
> +    }
> +}
> +
> +// SAFETY: We capture the lifetime of `Self` when we create a `PinHrTimerHandle`,
> +// so `Self` will outlive the handle.
> +unsafe impl<'a, T> UnsafeHrTimerPointer for Pin<&'a T>
> +where
> +    T: Send + Sync,
> +    T: HasHrTimer<T>,
> +    T: HrTimerCallback<Pointer<'a> = Self>,
> +    Pin<&'a T>: RawHrTimerCallback<CallbackTarget<'a> = Self>,
> +{
> +    type TimerHandle = PinHrTimerHandle<'a, T>;
> +
> +    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle {
> +        // Cast to pointer
> +        let self_ptr: *const T = <Self as core::ops::Deref>::deref(&self);

Why use deref? `get_ref` seems much cleaner.

> +
> +        // SAFETY:
> +        //  - As we derive `self_ptr` from a reference above, it must point to a
> +        //    valid `T`.
> +        //  - We keep `self` alive by wrapping it in a handle below.
> +        unsafe { T::start(self_ptr, expires) };
> +
> +        PinHrTimerHandle { inner: self }
> +    }
> +}
> +
> +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

---
Cheers,
Benno

> +        let receiver_ref = unsafe { &*receiver_ptr };
> +
> +        // SAFETY: `receiver_ref` only exists as pinned, so it is safe to pin it
> +        // here.
> +        let receiver_pin = unsafe { Pin::new_unchecked(receiver_ref) };
> +
> +        T::run(receiver_pin).into_c()
> +    }
> +}
>
> --
> 2.47.0