Re: [PATCH v10 10/13] rust: hrtimer: implement `HrTimerPointer` for `Pin<Box<T>>`

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

On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
> Allow `Pin<Box<T>>` to be the target of a timer callback.
>
> 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      |   3 ++
>  rust/kernel/time/hrtimer/tbox.rs | 109 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 112 insertions(+)
>
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index d2791fd624b7..991d37b0524a 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -443,3 +443,6 @@ unsafe fn timer_container_of(ptr: *mut $crate::time::hrtimer::HrTimer<$timer_typ
>  pub use pin::PinHrTimerHandle;
>  mod pin_mut;
>  pub use pin_mut::PinMutHrTimerHandle;
> +// `box` is a reserved keyword, so prefix with `t` for timer
> +mod tbox;
> +pub use tbox::BoxHrTimerHandle;
> diff --git a/rust/kernel/time/hrtimer/tbox.rs b/rust/kernel/time/hrtimer/tbox.rs
> new file mode 100644
> index 000000000000..a3b2ed849050
> --- /dev/null
> +++ b/rust/kernel/time/hrtimer/tbox.rs
> @@ -0,0 +1,109 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use super::HasHrTimer;
> +use super::HrTimer;
> +use super::HrTimerCallback;
> +use super::HrTimerHandle;
> +use super::HrTimerPointer;
> +use super::RawHrTimerCallback;
> +use crate::prelude::*;
> +use crate::time::Ktime;
> +use core::mem::ManuallyDrop;
> +use core::ptr::NonNull;
> +
> +/// A handle for a [`Box<HasHrTimer<T>>`] returned by a call to
> +/// [`HrTimerPointer::start`].
> +pub struct BoxHrTimerHandle<T, A>

Should this type implement `Send` and `Sync` depending on `T`?

> +where
> +    T: HasHrTimer<T>,
> +    A: crate::alloc::Allocator,
> +{
> +    pub(crate) inner: NonNull<T>,
> +    _p: core::marker::PhantomData<A>,
> +}
> +
> +// SAFETY: We implement drop below, and we cancel the timer in the drop
> +// implementation.
> +unsafe impl<T, A> HrTimerHandle for BoxHrTimerHandle<T, A>
> +where
> +    T: HasHrTimer<T>,
> +    A: crate::alloc::Allocator,
> +{
> +    fn cancel(&mut self) -> bool {
> +        // SAFETY: As we obtained `self.inner` from a valid reference when we
> +        // created `self`, it must point to a valid `T`.
> +        let timer_ptr = unsafe { <T as HasHrTimer<T>>::raw_get_timer(self.inner.as_ptr()) };
> +
> +        // SAFETY: As `timer_ptr` points into `T` and `T` is valid, `timer_ptr`
> +        // must point to a valid `HrTimer` instance.
> +        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
> +    }
> +}
> +
> +impl<T, A> Drop for BoxHrTimerHandle<T, A>
> +where
> +    T: HasHrTimer<T>,
> +    A: crate::alloc::Allocator,
> +{
> +    fn drop(&mut self) {
> +        self.cancel();
> +        // SAFETY: `self.inner` came from a `Box::into_raw` call

Please add this as an invariant to `Self`.

> +        drop(unsafe { Box::<T, A>::from_raw(self.inner.as_ptr()) })
> +    }
> +}
> +
> +impl<T, A> HrTimerPointer for Pin<Box<T, A>>
> +where
> +    T: 'static,
> +    T: Send + Sync,
> +    T: HasHrTimer<T>,
> +    T: for<'a> HrTimerCallback<Pointer<'a> = Pin<Box<T, A>>>,
> +    Pin<Box<T, A>>: for<'a> RawHrTimerCallback<CallbackTarget<'a> = Pin<&'a T>>,

I don't think this is necessary.

> +    A: crate::alloc::Allocator,
> +{
> +    type TimerHandle = BoxHrTimerHandle<T, A>;
> +
> +    fn start(self, expires: Ktime) -> Self::TimerHandle {
> +        // SAFETY:
> +        //  - We will not move out of this box during timer callback (we pass an
> +        //    immutable reference to the callback).
> +        //  - `Box::into_raw` is guaranteed to return a valid pointer.
> +        let inner =
> +            unsafe { NonNull::new_unchecked(Box::into_raw(Pin::into_inner_unchecked(self))) };
> +
> +        // 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(inner.as_ptr(), expires) };
> +
> +        BoxHrTimerHandle {
> +            inner,
> +            _p: core::marker::PhantomData,
> +        }
> +    }
> +}
> +
> +impl<T, A> RawHrTimerCallback for Pin<Box<T, A>>
> +where
> +    T: 'static,
> +    T: HasHrTimer<T>,
> +    T: for<'a> HrTimerCallback<Pointer<'a> = Pin<Box<T, A>>>,
> +    A: crate::alloc::Allocator,
> +{
> +    type CallbackTarget<'a> = Pin<&'a T>;

Why isn't this `Pin<&'a mut 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: We called `Box::into_raw` when we queued the timer.
> +        let tbox = ManuallyDrop::new(Box::into_pin(unsafe { Box::<T, A>::from_raw(data_ptr) }));

Since you turn this into a reference below and never run the drop, why
not turn the pointer directly into a reference?

---
Cheers,
Benno

> +
> +        T::run(tbox.as_ref()).into_c()
> +    }
> +}
>
> --
> 2.47.0