linux-kernel - Re: [PATCH v2 8/8] rust: hrtimer: Add HrTimer::expires()

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Message-ID: <87cyd3kp56.fsf@kernel.org>
Date: Wed, 23 Apr 2025 15:01:41 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Lyude Paul <lyude@...hat.com>
Cc: rust-for-linux@...r.kernel.org,  linux-kernel@...r.kernel.org,  Boqun
 Feng <boqun.feng@...il.com>,  FUJITA Tomonori <fujita.tomonori@...il.com>,
  Frederic Weisbecker <frederic@...nel.org>,  Thomas Gleixner
 <tglx@...utronix.de>,  Anna-Maria Behnsen <anna-maria@...utronix.de>,
  John Stultz <jstultz@...gle.com>,  Stephen Boyd <sboyd@...nel.org>,
  Miguel Ojeda <ojeda@...nel.org>,  Alex Gaynor <alex.gaynor@...il.com>,
  Gary Guo <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,
  Benno Lossin <benno.lossin@...ton.me>,  Alice Ryhl
 <aliceryhl@...gle.com>,  Trevor Gross <tmgross@...ch.edu>,  Danilo
 Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 8/8] rust: hrtimer: Add HrTimer::expires()

Lyude Paul <lyude@...hat.com> writes:

> This adds the ability to read the expiry time of the given timer.
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
>
> ---
> V2:
> * Convert from Ktime to Instant
> * Use read_volatile instead of read and add a FIXME
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
>  rust/kernel/time/hrtimer.rs | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index c84dcdacb4882..b8a74c15e6609 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -73,7 +73,10 @@
>      time::{Delta, Instant},
>      types::Opaque,
>  };
> -use core::{marker::PhantomData, ptr::NonNull};
> +use core::{
> +    marker::PhantomData,
> +    ptr::{addr_of, NonNull},
> +};
>  use pin_init::PinInit;
>  
>  /// A timer backed by a C `struct hrtimer`.
> @@ -136,7 +139,7 @@ unsafe fn raw_get(this: *const Self) -> *mut bindings::hrtimer {
>          // SAFETY: The field projection to `timer` does not go out of bounds,
>          // because the caller of this function promises that `this` points to an
>          // allocation of at least the size of `Self`.
> -        unsafe { Opaque::raw_get(core::ptr::addr_of!((*this).timer)) }
> +        unsafe { Opaque::raw_get(addr_of!((*this).timer)) }
>      }
>  
>      /// Cancel an initialized and potentially running timer.
> @@ -225,6 +228,21 @@ pub fn forward(&mut self, now: Instant, duration: Delta) -> u64 {
>      pub fn forward_now(&mut self, duration: Delta) -> u64 {
>          self.forward(self.clock_base().time(), duration)
>      }
> +
> +    /// Return the time expiry for this [`HrTimer`].
> +    ///
> +    /// This value should only be used as a snapshot, as the actual expiry time could change after
> +    /// this function is called.
> +    pub fn expires(&self) -> Instant {
> +        // SAFETY: `self` is an immutable reference and thus always points to a valid `HrTimer`.
> +        let c_timer_ptr = unsafe { HrTimer::raw_get(self) };
> +
> +        // SAFETY: There's no actual locking here, a racy read is fine and expected.
> +        Instant::from_nanos(unsafe {
> +            // FIXME: read_volatile

Please adopt similar wording as in `file.rs`:

        // This `read_volatile` is intended to correspond to a READ_ONCE call.
        // FIXME(read_once): Replace with `read_once` when available on the Rust side.


Best regards,
Andreas Hindborg