Re: [PATCH v10 07/13] rust: hrtimer: implement `UnsafeHrTimerPointer` for `Pin<&T>`

[Andreas Hindborg <a.hindborg@kernel.org>]

"Benno Lossin" <benno.lossin@proton.me> writes:

> On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
>> Allow pinned references to structs that contain a `HrTimer` node to be
>> scheduled with the `hrtimer` subsystem.
>>
>> 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     |  2 +
>>  rust/kernel/time/hrtimer/pin.rs | 99 +++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 101 insertions(+)
>>
>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>> index d90a25785f87..2ca56397eade 100644
>> --- a/rust/kernel/time/hrtimer.rs
>> +++ b/rust/kernel/time/hrtimer.rs
>> @@ -439,3 +439,5 @@ unsafe fn timer_container_of(ptr: *mut $crate::time::hrtimer::HrTimer<$timer_typ
>>
>>  mod arc;
>>  pub use arc::ArcHrTimerHandle;
>> +mod pin;
>> +pub use pin::PinHrTimerHandle;
>> diff --git a/rust/kernel/time/hrtimer/pin.rs b/rust/kernel/time/hrtimer/pin.rs
>> new file mode 100644
>> index 000000000000..6c9f2190f8e1
>> --- /dev/null
>> +++ b/rust/kernel/time/hrtimer/pin.rs
>> @@ -0,0 +1,99 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +use super::HasHrTimer;
>> +use super::HrTimer;
>> +use super::HrTimerCallback;
>> +use super::HrTimerHandle;
>> +use super::RawHrTimerCallback;
>> +use super::UnsafeHrTimerPointer;
>> +use crate::time::Ktime;
>> +use core::pin::Pin;
>> +
>> +/// A handle for a `Pin<&HasHrTimer>`. When the handle exists, the timer might be
>> +/// running.
>> +pub struct PinHrTimerHandle<'a, T>
>> +where
>> +    T: HasHrTimer<T>,
>> +{
>> +    pub(crate) inner: Pin<&'a T>,
>> +}
>> +
>> +// 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, T> HrTimerHandle for PinHrTimerHandle<'a, T>
>> +where
>> +    T: HasHrTimer<T>,
>> +{
>> +    fn cancel(&mut self) -> bool {
>> +        let self_ptr: *const T = self.inner.get_ref();
>> +
>> +        // SAFETY: As we got `self_ptr` from a reference above, it must point to
>> +        // a valid `T`.
>> +        let timer_ptr = unsafe { <T as HasHrTimer<T>>::raw_get_timer(self_ptr) };
>> +
>> +        // SAFETY: As `timer_ptr` is derived from a reference, it must point to
>> +        // a valid and initialized `HrTimer`.
>> +        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
>> +    }
>> +}
>> +
>> +impl<'a, T> Drop for PinHrTimerHandle<'a, T>
>> +where
>> +    T: HasHrTimer<T>,
>> +{
>> +    fn drop(&mut self) {
>> +        self.cancel();
>> +    }
>> +}
>> +
>> +// SAFETY: We capture the lifetime of `Self` when we create a `PinHrTimerHandle`,
>> +// so `Self` will outlive the handle.
>> +unsafe impl<'a, T> UnsafeHrTimerPointer for Pin<&'a T>
>> +where
>> +    T: Send + Sync,
>> +    T: HasHrTimer<T>,
>> +    T: HrTimerCallback<Pointer<'a> = Self>,
>> +    Pin<&'a T>: RawHrTimerCallback<CallbackTarget<'a> = Self>,
>> +{
>> +    type TimerHandle = PinHrTimerHandle<'a, T>;
>> +
>> +    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle {
>> +        // Cast to pointer
>> +        let self_ptr: *const T = <Self as core::ops::Deref>::deref(&self);
>
> Why use deref? `get_ref` seems much cleaner.

Sure.

>
>> +
>> +        // SAFETY:
>> +        //  - As we derive `self_ptr` from a reference above, it must point to a
>> +        //    valid `T`.
>> +        //  - We keep `self` alive by wrapping it in a handle below.
>> +        unsafe { T::start(self_ptr, expires) };
>> +
>> +        PinHrTimerHandle { inner: self }
>> +    }
>> +}
>> +
>> +impl<'a, T> RawHrTimerCallback for Pin<&'a T>
>> +where
>> +    T: HasHrTimer<T>,
>> +    T: HrTimerCallback<Pointer<'a> = Self>,
>> +{
>> +    type CallbackTarget<'b> = Self;
>> +
>> +    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
>> +        // `HrTimer` is `repr(C)`
>> +        let timer_ptr = ptr as *mut HrTimer<T>;
>> +
>> +        // SAFETY: By the safety requirement of this function, `timer_ptr`
>> +        // points to a `HrTimer<T>` contained in an `T`.
>> +        let receiver_ptr = unsafe { T::timer_container_of(timer_ptr) };
>> +
>> +        // SAFETY: By the safety requirement of this function, `timer_ptr`
>> +        // points to a `HrTimer<T>` contained in an `T`.
>
> This justification seems wrong it talks about `HrTimer<T>`, but here we
> have a `*const T`... Also see [1] (I am mainly interested in your
> justification for the lifetime).
>
> [1]: https://doc.rust-lang.org/std/ptr/index.html#pointer-to-reference-conversion

How is this:

        // SAFETY:
        //  - By the safety requirement of this function, `timer_ptr`
        //    points to a `HrTimer<T>` contained in an `T`.
        //  - The `PinHrTimerHandle` associated with this timer is guaranteed to
        //    be alive until this method returns. As the handle borrows from
        //    `T`, `T` is also guaranteed to be alive for the duration of this
        //    function.


Best regards,
Andreas Hindborg