Re: [PATCH v3 08/13] rust: hrtimer: implement `UnsafeTimerPointer` for `Pin<&mut T>`

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-10-17 at 15:04 +0200, Andreas Hindborg wrote:
> Allow pinned mutable references to structs that contain a `Timer` node to
> be scheduled with the `hrtimer` subsystem.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/hrtimer.rs         |  1 +
>  rust/kernel/hrtimer/pin_mut.rs | 99 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 100 insertions(+)
> 
> diff --git a/rust/kernel/hrtimer.rs b/rust/kernel/hrtimer.rs
> index ceedf330a803ec2db7ff6c25713ae48e2fd1f4ca..940390aa2a6c6a222534b545d8d2fbd639a19f64 100644
> --- a/rust/kernel/hrtimer.rs
> +++ b/rust/kernel/hrtimer.rs
> @@ -363,3 +363,4 @@ unsafe fn raw_get_timer(ptr: *const Self) ->
>  
>  mod arc;
>  mod pin;
> +mod pin_mut;
> diff --git a/rust/kernel/hrtimer/pin_mut.rs b/rust/kernel/hrtimer/pin_mut.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..d6c85647a762066776cf292620a8b19c12721876
> --- /dev/null
> +++ b/rust/kernel/hrtimer/pin_mut.rs
> @@ -0,0 +1,99 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use super::HasTimer;
> +use super::RawTimerCallback;
> +use super::Timer;
> +use super::TimerCallback;
> +use super::TimerHandle;
> +use super::UnsafeTimerPointer;
> +use crate::time::Ktime;
> +use core::pin::Pin;
> +
> +/// A handle for a `Pin<&mut HasTimer>`. When the handle exists, the timer might
> +/// be running.
> +pub struct PinMutTimerHandle<'a, U>
> +where
> +    U: HasTimer<U>,
> +{
> +    pub(crate) inner: Pin<&'a mut U>,
> +}
> +
> +// 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, U> TimerHandle for PinMutTimerHandle<'a, U>
> +where
> +    U: HasTimer<U>,
> +{
> +    fn cancel(&mut self) -> bool {
> +        // SAFETY: We are not moving out of `self` or handing out mutable
> +        // references to `self`.
> +        let self_ptr = unsafe { self.inner.as_mut().get_unchecked_mut() as *mut U };
> +
> +        // SAFETY: As we got `self_ptr` from a 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` is derived from a reference, it must point to
> +        // a valid and initialized `Timer`.
> +        unsafe { Timer::<U>::raw_cancel(timer_ptr) }
> +    }
> +}
> +
> +impl<'a, U> Drop for PinMutTimerHandle<'a, U>
> +where
> +    U: HasTimer<U>,
> +{
> +    fn drop(&mut self) {
> +        self.cancel();
> +    }
> +}
> +
> +// SAFETY: We capture the lifetime of `Self` when we create a
> +// `PinMutTimerHandle`, so `Self` will outlive the handle.
> +unsafe impl<'a, U> UnsafeTimerPointer for Pin<&'a mut U>
> +where
> +    U: Send + Sync,
> +    U: HasTimer<U>,
> +    U: TimerCallback<CallbackTarget<'a> = Self>,
> +{
> +    type TimerHandle = PinMutTimerHandle<'a, U>;
> +
> +    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle {
> +        use core::ops::Deref;

Same strange deref use here

> +
> +        // Cast to pointer
> +        let self_ptr = self.deref() as *const U;
> +
> +        // SAFETY: As we derive `self_ptr` from a reference above, it must point
> +        // to a valid `U`.
> +        unsafe { U::start(self_ptr, expires) };
> +
> +        PinMutTimerHandle { inner: self }
> +    }
> +}
> +
> +impl<'a, U> RawTimerCallback for Pin<&'a mut U>
> +where
> +    U: HasTimer<U>,
> +    U: TimerCallback<CallbackTarget<'a> = Self>,
> +    U: TimerCallback<CallbackTargetParameter<'a> = Self>,
> +{
> +    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
> +        // `Timer` is `repr(C)`
> +        let timer_ptr = ptr as *mut Timer<U>;
> +
> +        // SAFETY: By the safety requirement of this function, `timer_ptr`
> +        // points to a `Timer<U>` contained in an `U`.
> +        let receiver_ptr = unsafe { U::timer_container_of(timer_ptr) };
> +
> +        // SAFETY: By the safety requirement of this function, `timer_ptr`
> +        // points to a `Timer<U>` contained in an `U`.
> +        let receiver_ref = unsafe { &mut *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) };
> +
> +        U::run(receiver_pin).into()
> +    }
> +}
> 

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

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