Re: [PATCH v9 06/13] rust: hrtimer: add `hrtimer::ScopedHrTimerPointer`

[Lyude Paul <lyude@redhat.com>]

Reviewed-by: Lyude Paul <lyude@redhat.com>

On Mon, 2025-02-24 at 13:03 +0100, Andreas Hindborg wrote:
> Add the trait `ScopedHrTimerPointer` to allow safe use of stack allocated
> timers. Safety is achieved by pinning the stack in place while timers are
> running.
> 
> Implement the trait for all types that implement `UnsafeHrTimerPointer`.
> 
> Acked-by: Frederic Weisbecker <frederic@kernel.org>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/time/hrtimer.rs | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index 2cb40b011673..64b769ad59cc 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -212,6 +212,39 @@ pub unsafe trait UnsafeHrTimerPointer: Sync + Sized {
>      unsafe fn start(self, expires: Ktime) -> Self::TimerHandle;
>  }
>  
> +/// A trait for stack allocated timers.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that `start_scoped` does not return until the
> +/// timer is dead and the timer handler is not running.
> +pub unsafe trait ScopedHrTimerPointer {
> +    /// Start the timer to run after `expires` time units and immediately
> +    /// after call `f`. When `f` returns, the timer is cancelled.
> +    fn start_scoped<T, F>(self, expires: Ktime, f: F) -> T
> +    where
> +        F: FnOnce() -> T;
> +}
> +
> +// SAFETY: By the safety requirement of [`UnsafeHrTimerPointer`], dropping the
> +// handle returned by [`UnsafeHrTimerPointer::start`] ensures that the timer is
> +// killed.
> +unsafe impl<T> ScopedHrTimerPointer for T
> +where
> +    T: UnsafeHrTimerPointer,
> +{
> +    fn start_scoped<U, F>(self, expires: Ktime, f: F) -> U
> +    where
> +        F: FnOnce() -> U,
> +    {
> +        // SAFETY: We drop the timer handle below before returning.
> +        let handle = unsafe { UnsafeHrTimerPointer::start(self, expires) };
> +        let t = f();
> +        drop(handle);
> +        t
> +    }
> +}
> +
>  /// 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.
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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