Message-ID: <D8A28FTNL1BC.3AYZMT4OYF79R@proton.me>
Date: Fri, 07 Mar 2025 13:15:26 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Danilo Krummrich <dakr@...nel.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Lyude Paul <lyude@...hat.com>, Guangbo Cui <2407018371@...com>, Dirk Behme <dirk.behme@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, Tamir Duberstein <tamird@...il.com>, Markus Elfring <Markus.Elfring@....de>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 08/13] rust: hrtimer: implement `UnsafeHrTimerPointer` for `Pin<&mut T>`

On Fri Mar 7, 2025 at 11:11 AM CET, Andreas Hindborg wrote:
> Allow pinned mutable references to structs that contain a `HrTimer` node to
> be scheduled with the `hrtimer` subsystem.
>
> Acked-by: Frederic Weisbecker <frederic@...nel.org>
> Reviewed-by: Lyude Paul <lyude@...hat.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
>  rust/kernel/time/hrtimer.rs         |   2 +
>  rust/kernel/time/hrtimer/pin_mut.rs | 101 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 103 insertions(+)
>
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index 2ca56397eade..d2791fd624b7 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -441,3 +441,5 @@ unsafe fn timer_container_of(ptr: *mut $crate::time::hrtimer::HrTimer<$timer_typ
>  pub use arc::ArcHrTimerHandle;
>  mod pin;
>  pub use pin::PinHrTimerHandle;
> +mod pin_mut;
> +pub use pin_mut::PinMutHrTimerHandle;
> diff --git a/rust/kernel/time/hrtimer/pin_mut.rs b/rust/kernel/time/hrtimer/pin_mut.rs
> new file mode 100644
> index 000000000000..4f4a9e9602d8
> --- /dev/null
> +++ b/rust/kernel/time/hrtimer/pin_mut.rs
> @@ -0,0 +1,101 @@
> +// 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<&mut HasHrTimer>`. When the handle exists, the timer might
> +/// be running.
> +pub struct PinMutHrTimerHandle<'a, T>
> +where
> +    T: HasHrTimer<T>,
> +{
> +    pub(crate) inner: Pin<&'a mut 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 PinMutHrTimerHandle<'a, T>
> +where
> +    T: HasHrTimer<T>,
> +{
> +    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 T };
> +
> +        // 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 PinMutHrTimerHandle<'a, T>
> +where
> +    T: HasHrTimer<T>,
> +{
> +    fn drop(&mut self) {
> +        self.cancel();
> +    }
> +}
> +
> +// SAFETY: We capture the lifetime of `Self` when we create a
> +// `PinMutHrTimerHandle`, so `Self` will outlive the handle.
> +unsafe impl<'a, T> UnsafeHrTimerPointer for Pin<&'a mut T>
> +where
> +    T: Send + Sync,
> +    T: HasHrTimer<T>,
> +    T: HrTimerCallback<Pointer<'a> = Self>,
> +    Pin<&'a mut T>: RawHrTimerCallback<CallbackTarget<'a> = Self>,
> +{
> +    type TimerHandle = PinMutHrTimerHandle<'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);

You cannot go through a shared reference here, since you convert the
pointer obtained here in the `run` function later back into a mutable
reference. You will have to use `get_unchecked_mut` or
`into_inner_unchecked`.

> +
> +        // 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) };
> +
> +        PinMutHrTimerHandle { inner: self }
> +    }
> +}
> +
> +impl<'a, T> RawHrTimerCallback for Pin<&'a mut 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`.

Same question here as for the `Pin<&T>` patch.

---
Cheers,
Benno

> +        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) };
> +
> +        T::run(receiver_pin).into_c()
> +    }
> +}
>
> --
> 2.47.0

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