Re: [PATCH v8 06/14] rust: hrtimer: add `UnsafeHrTimerPointer`

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

On 18.02.25 14:27, Andreas Hindborg wrote:
> Add a trait to allow unsafely queuing stack allocated timers.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

> ---
>  rust/kernel/time/hrtimer.rs | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index e342193f985eb..196794089f033 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -157,6 +157,37 @@ pub trait HrTimerPointer: Sync + Sized {
>      fn start(self, expires: Ktime) -> Self::TimerHandle;
>  }
> 
> +/// Unsafe version of [`HrTimerPointer`] for situations where leaking the
> +/// [`HrTimerHandle`] returned by `start` would be unsound. This is the case for
> +/// stack allocated timers.
> +///
> +/// Typical implementers are pinned references such as [`Pin<&T>`].
> +///
> +/// # Safety
> +///
> +/// Implementers of this trait must ensure that instances of types implementing
> +/// [`UnsafeHrTimerPointer`] outlives any associated [`HrTimerPointer::TimerHandle`]
> +/// instances.
> +pub unsafe trait UnsafeHrTimerPointer: Sync + Sized {
> +    /// A handle representing a running timer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// If the timer is running, or if the timer callback is executing when the
> +    /// handle is dropped, the drop method of [`Self::TimerHandle`] must not return
> +    /// until the timer is stopped and the callback has completed.
> +    type TimerHandle: HrTimerHandle;
> +
> +    /// Start the timer after `expires` time units. If the timer was already
> +    /// running, it is restarted at the new expiry time.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Caller promises keep the timer structure alive until the timer is dead.
> +    /// Caller can ensure this by not leaking the returned [`Self::TimerHandle`].
> +    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle;

I have an interesting idea to make this function safe, but I don't want
to block this series.

How about we have the following signature:

    fn start(self, expires: Ktime) -> impl PinInit<Self::TimerHandle>;

And in the safety requirements of the trait we ask that `TimerHandle` be
`!Unpin`. Then the TimerHandle cannot be forgotten because it is pinned
and pinned values have the drop guarantee [1].

You can then use `stack_pin_init!` to pin the value directly on the
stack (dropping it by leaving its scope will then cancel the timer &
possibly wait for the timer callback to finish running).

[1]: https://doc.rust-lang.org/std/pin/index.html#drop-guarantee

---
Cheers,
Benno

> +}
> +
>  /// Implemented by [`HrTimerPointer`] implementers to give the C timer callback a
>  /// function to call.
>  // This is split from `HrTimerPointer` to make it easier to specify trait bounds.
> 
> --
> 2.47.0
> 
>