Re: [PATCH 2/6] rust: hrtimer: Add HrTimerCallbackContext and ::forward()

From: Andreas Hindborg
Date: Tue Apr 08 2025 - 08:25:03 EST

"Lyude Paul" <lyude@xxxxxxxxxx> writes:

> With Linux's hrtimer API, certain functions require we either acquire
> proper locking to call specific methods - or that we call said methods from
> the context of the timer callback. hrtimer_forward() is one of these
> functions, so we start by adding a new HrTimerCallbackContext type which
> provides a way of calling these methods that is inaccessible outside of
> hrtimer callbacks.

Based on tglx comment, we should be able to call this function if the
timer is stopped and we have a unique ownership of the timer. Do you
want to add that? If not, could you add a note about this somewhere?

>
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> ---
> rust/kernel/time/hrtimer.rs | 50 +++++++++++++++++++++++++++--
> rust/kernel/time/hrtimer/arc.rs | 7 +++-
> rust/kernel/time/hrtimer/pin.rs | 7 +++-
> rust/kernel/time/hrtimer/pin_mut.rs | 9 ++++--
> rust/kernel/time/hrtimer/tbox.rs | 7 +++-
> 5 files changed, 73 insertions(+), 7 deletions(-)
>
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index 4fc49f1931259..c92b10524f892 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -69,7 +69,7 @@
>
> use super::ClockId;
> use crate::{init::PinInit, prelude::*, time::Ktime, types::Opaque};
> -use core::marker::PhantomData;
> +use core::{marker::PhantomData, ptr::NonNull};
>
> /// A timer backed by a C `struct hrtimer`.
> ///
> @@ -279,7 +279,10 @@ pub trait HrTimerCallback {
> type Pointer<'a>: RawHrTimerCallback;
>
> /// Called by the timer logic when the timer fires.
> - fn run(this: <Self::Pointer<'_> as RawHrTimerCallback>::CallbackTarget<'_>) -> HrTimerRestart
> + fn run<T>(
> + this: <Self::Pointer<'_> as RawHrTimerCallback>::CallbackTarget<'_>,
> + ctx: HrTimerCallbackContext<'_, T>
> + ) -> HrTimerRestart
> where
> Self: Sized;
> }
> @@ -470,6 +473,49 @@ fn into_c(self) -> bindings::hrtimer_mode {
> }
> }
>
> +/// Privileged smart-pointer for a [`HrTimer`] callback context.
> +///
> +/// This provides access to various methods for a [`HrTimer`] which can only be safely called within
> +/// its callback.
> +///
> +/// # Invariants
> +///
> +/// * The existence of this type means the caller is currently within the callback for a
> +/// [`HrTimer`].
> +/// * `self.0` always points to a live instance of [`HrTimer<T>`].
> +pub struct HrTimerCallbackContext<'a, T>(NonNull<HrTimer<T>>, PhantomData<&'a ()>);
> +
> +impl<'a, T> HrTimerCallbackContext<'a, T> {
> + /// Create a new [`HrTimerCallbackContext`].
> + ///
> + /// # Safety
> + ///
> + /// This function relies on the caller being within the context of a timer callback, so it must
> + /// not be used anywhere except for within implementations of [`RawHrTimerCallback::run`]. The
> + /// caller promises that `timer` points to a valid initialized instance of
> + /// [`bindings::hrtimer`].
> + pub(crate) unsafe fn from_raw(timer: *mut HrTimer<T>) -> Self {
> + // SAFETY: The caller guarantees `timer` is a valid pointer to an initialized
> + // `bindings::hrtimer`

Missing `// INVARIANT` comment.

> + Self(unsafe { NonNull::new_unchecked(timer) }, PhantomData)
> + }
> +
> + /// Get the raw `bindings::hrtimer` pointer for this [`HrTimerCallbackContext`].
> + pub(crate) fn raw_get_timer(&self) -> *mut bindings::hrtimer {
> + // SAFETY: By our type invariants, `self.0` always points to a valid [`HrTimer<T>`].
> + unsafe { HrTimer::raw_get(self.0.as_ptr()) }
> + }
> +
> + /// Forward the timer expiry so it will expire in the future.
> + ///
> + /// Note that this does not requeue the timer, it simply updates its expiry value. It returns
> + /// the number of overruns that have occurred as a result of the expiry change.
> + pub fn forward(&self, now: Ktime, interval: Ktime) -> u64 {
> + // SAFETY: The C API requirements for this function are fulfilled by our type invariants.
> + unsafe { bindings::hrtimer_forward(self.raw_get_timer(), now.to_ns(), interval.to_ns()) }
> + }
> +}
> +
> /// Use to implement the [`HasHrTimer<T>`] trait.
> ///
> /// See [`module`] documentation for an example.
> diff --git a/rust/kernel/time/hrtimer/arc.rs b/rust/kernel/time/hrtimer/arc.rs
> index 4a984d85b4a10..7dd9f46a0720d 100644
> --- a/rust/kernel/time/hrtimer/arc.rs
> +++ b/rust/kernel/time/hrtimer/arc.rs
> @@ -3,6 +3,7 @@
> use super::HasHrTimer;
> use super::HrTimer;
> use super::HrTimerCallback;
> +use super::HrTimerCallbackContext;
> use super::HrTimerHandle;
> use super::HrTimerPointer;
> use super::RawHrTimerCallback;
> @@ -95,6 +96,10 @@ impl<T> RawHrTimerCallback for Arc<T>
> // allocation from other `Arc` clones.
> let receiver = unsafe { ArcBorrow::from_raw(data_ptr) };
>
> - T::run(receiver).into_c()
> + // 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`.

This safety comment does not match the safety requirements for the
unsafe fn we call.

> + let context = unsafe { HrTimerCallbackContext::from_raw(timer_ptr) };
> +
> + T::run(receiver, context).into_c()
> }
> }
> diff --git a/rust/kernel/time/hrtimer/pin.rs b/rust/kernel/time/hrtimer/pin.rs
> index f760db265c7b5..a8e1b76bf0736 100644
> --- a/rust/kernel/time/hrtimer/pin.rs
> +++ b/rust/kernel/time/hrtimer/pin.rs
> @@ -3,6 +3,7 @@
> use super::HasHrTimer;
> use super::HrTimer;
> use super::HrTimerCallback;
> +use super::HrTimerCallbackContext;
> use super::HrTimerHandle;
> use super::RawHrTimerCallback;
> use super::UnsafeHrTimerPointer;
> @@ -99,6 +100,10 @@ impl<'a, T> RawHrTimerCallback for Pin<&'a T>
> // here.
> let receiver_pin = unsafe { Pin::new_unchecked(receiver_ref) };
>
> - T::run(receiver_pin).into_c()
> + // 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`.

Same as above.

> + let context = unsafe { HrTimerCallbackContext::from_raw(timer_ptr) };
> +
> + T::run(receiver_pin, context).into_c()
> }
> }
> diff --git a/rust/kernel/time/hrtimer/pin_mut.rs b/rust/kernel/time/hrtimer/pin_mut.rs
> index 90c0351d62e4b..2dd2ebfd7efaf 100644
> --- a/rust/kernel/time/hrtimer/pin_mut.rs
> +++ b/rust/kernel/time/hrtimer/pin_mut.rs
> @@ -1,7 +1,8 @@
> // SPDX-License-Identifier: GPL-2.0
>
> use super::{
> - HasHrTimer, HrTimer, HrTimerCallback, HrTimerHandle, RawHrTimerCallback, UnsafeHrTimerPointer,
> + HasHrTimer, HrTimer, HrTimerCallback, HrTimerCallbackContext, HrTimerHandle, RawHrTimerCallback,
> + UnsafeHrTimerPointer,
> };
> use crate::time::Ktime;
> use core::{marker::PhantomData, pin::Pin, ptr::NonNull};
> @@ -103,6 +104,10 @@ impl<'a, T> RawHrTimerCallback for Pin<&'a mut T>
> // here.
> let receiver_pin = unsafe { Pin::new_unchecked(receiver_ref) };
>
> - T::run(receiver_pin).into_c()
> + // 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`.

Again.

> + let context = unsafe { HrTimerCallbackContext::from_raw(timer_ptr) };
> +
> + T::run(receiver_pin, context).into_c()
> }
> }
> diff --git a/rust/kernel/time/hrtimer/tbox.rs b/rust/kernel/time/hrtimer/tbox.rs
> index 2071cae072342..e3214f7173beb 100644
> --- a/rust/kernel/time/hrtimer/tbox.rs
> +++ b/rust/kernel/time/hrtimer/tbox.rs
> @@ -3,6 +3,7 @@
> use super::HasHrTimer;
> use super::HrTimer;
> use super::HrTimerCallback;
> +use super::HrTimerCallbackContext;
> use super::HrTimerHandle;
> use super::HrTimerPointer;
> use super::RawHrTimerCallback;
> @@ -115,6 +116,10 @@ impl<T, A> RawHrTimerCallback for Pin<Box<T, A>>
> // `data_ptr` exist.
> let data_mut_ref = unsafe { Pin::new_unchecked(&mut *data_ptr) };
>
> - T::run(data_mut_ref).into_c()
> + // 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`.

Also here.

> + let context = unsafe { HrTimerCallbackContext::from_raw(timer_ptr) };
> +
> + T::run(data_mut_ref, context).into_c()
> }
> }


Best regards,
Andreas Hindborg