Re: [PATCH v3 04/13] rust: hrtimer: implement `TimerPointer` for `Arc`

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-10-17 at 15:04 +0200, Andreas Hindborg wrote:
> This patch allows the use of intrusive `hrtimer` fields in structs that are
> managed by an `Arc`.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/hrtimer.rs     |  3 +-
>  rust/kernel/hrtimer/arc.rs | 89 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 91 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/hrtimer.rs b/rust/kernel/hrtimer.rs
> index 99058a690f2e18b5c26c94c71133407019aa4a26..6427b0450c694105190c8cddea0c768ab195aca2 100644
> --- a/rust/kernel/hrtimer.rs
> +++ b/rust/kernel/hrtimer.rs
> @@ -107,7 +107,6 @@ unsafe fn raw_get(ptr: *const Self) -> *mut bindings::hrtimer {
>      /// # Safety
>      ///
>      /// `self_ptr` must point to a valid `Self`.
> -    #[allow(dead_code)]
>      pub(crate) unsafe fn raw_cancel(self_ptr: *const Self) -> bool {
>          // SAFETY: timer_ptr points to an allocation of at least `Timer` size.
>          let c_timer_ptr = unsafe { Timer::raw_get(self_ptr) };
> @@ -302,3 +301,5 @@ unsafe fn raw_get_timer(ptr: *const Self) ->
>          }
>      }
>  }
> +
> +mod arc;
> diff --git a/rust/kernel/hrtimer/arc.rs b/rust/kernel/hrtimer/arc.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..881de053ecad866a26e46a0123ec2bf38511c2bc
> --- /dev/null
> +++ b/rust/kernel/hrtimer/arc.rs
> @@ -0,0 +1,89 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use super::HasTimer;
> +use super::RawTimerCallback;
> +use super::Timer;
> +use super::TimerCallback;
> +use super::TimerHandle;
> +use super::TimerPointer;
> +use crate::sync::Arc;
> +use crate::sync::ArcBorrow;
> +use crate::time::Ktime;

Is there a reason you're using separate lines for each include instead of
grouping them together by module?

> +
> +/// A handle for an `Arc<HasTimer<U>>` returned by a call to
> +/// [`TimerPointer::start`].
> +pub struct ArcTimerHandle<U>
> +where
> +    U: HasTimer<U>,
> +{
> +    pub(crate) inner: Arc<U>,
> +}
> +
> +// SAFETY: We implement drop below, and we cancel the timer in the drop
> +// implementation.
> +unsafe impl<U> TimerHandle for ArcTimerHandle<U>
> +where
> +    U: HasTimer<U>,
> +{
> +    fn cancel(&mut self) -> bool {
> +        let self_ptr = Arc::as_ptr(&self.inner);
> +
> +        // SAFETY: As we obtained `self_ptr` from a valid reference above, it
> +        // must point to a valid `U`.
> +        let timer_ptr = unsafe { <U as HasTimer<U>>::raw_get_timer(self_ptr) };
> +
> +        // SAFETY: As `timer_ptr` points into `U` and `U` is valid, `timer_ptr`
> +        // must point to a valid `Timer` instance.
> +        unsafe { Timer::<U>::raw_cancel(timer_ptr) }
> +    }
> +}
> +
> +impl<U> Drop for ArcTimerHandle<U>
> +where
> +    U: HasTimer<U>,
> +{
> +    fn drop(&mut self) {
> +        self.cancel();
> +    }
> +}
> +
> +impl<U> TimerPointer for Arc<U>
> +where
> +    U: Send + Sync,
> +    U: HasTimer<U>,
> +    U: for<'a> TimerCallback<CallbackTarget<'a> = Self>,
> +{
> +    type TimerHandle = ArcTimerHandle<U>;
> +
> +    fn start(self, expires: Ktime) -> ArcTimerHandle<U> {
> +        // SAFETY: Since we generate the pointer passed to `start` from a
> +        // valid reference, it is a valid pointer.
> +        unsafe { U::start(Arc::as_ptr(&self), expires) };
> +
> +        ArcTimerHandle { inner: self }
> +    }
> +}
> +
> +impl<U> RawTimerCallback for Arc<U>
> +where
> +    U: HasTimer<U>,
> +    U: for<'a> TimerCallback<CallbackTarget<'a> = Self>,
> +    U: for<'a> TimerCallback<CallbackTargetParameter<'a> = ArcBorrow<'a, U>>,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
> +        // `Timer` is `repr(C)`
> +        let timer_ptr = ptr.cast::<kernel::hrtimer::Timer<U>>();
> +
> +        // SAFETY: By C API contract `ptr` is the pointer we passed when
> +        // queuing the timer, so it is a `Timer<T>` embedded in a `T`.
> +        let data_ptr = unsafe { U::timer_container_of(timer_ptr) };
> +
> +        // SAFETY: `data_ptr` points to the `U` that was used to queue the
> +        // timer. This `U` is contained in an `Arc`.
> +        let receiver = unsafe { ArcBorrow::from_raw(data_ptr) };
> +
> +        U::run(receiver);
> +
> +        bindings::hrtimer_restart_HRTIMER_NORESTART
> +    }
> +}
> 

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

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