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

[Lyude Paul <lyude@redhat.com>]

On Tue, 2025-04-08 at 13:47 +0200, Andreas Hindborg wrote:
> "Lyude Paul" <lyude@redhat.com> 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?


Happy to! So, I think if we were to add a function for this I assume we would
want something like this?

fn forward(&mut self, now: Instant, interval: Duration) -> u64 {
    self.cancel();
    /* Do actual forward stuff here */
}

Of course with some documentation pointing out that this function will stop
the timer if required.

> 
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > ---
> >  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
> 
> 

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

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