Re: [PATCH] rust: hrtimer: document handle based design rationale

[Boqun Feng <boqun@kernel.org>]

On Sun, Feb 15, 2026 at 09:36:04PM +0100, Andreas Hindborg wrote:
> Add implementation notes explaining why the hrtimer abstraction uses a
> handle based approach.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Thanks for adding this!

Reviewed-by: Boqun Feng <boqun@kernel.org>

However I feel these are not just implementation details. They are the
design decision we made because of the limitation you mentioned. Maybe
make them "//!" doc comment and just put them as a separate section that
describes the necessity of handles? Thoughts?

Regards,
Boqun

> ---
>  rust/kernel/time/hrtimer.rs | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index 856d2d929a008..f92880b2cbdbd 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -67,6 +67,18 @@
>  //! A `restart` operation on a timer in the **stopped** state is equivalent to a
>  //! `start` operation.
>  
> +// Implementation details



> +//
> +// The reasoning for adopting a handle based approach:
> +// - If we explicitly drop the target of a timer callback in the timer callback, we
> +//   may get a dangling reference.
> +// - If the callback owns the last reference to the target, target may be dropped
> +//   in non-sleepable context when the callback is finished.
> +// - When dropping an object that is the target of an armed timer, we may drop
> +//   fields accessed by the timer callback before we cancel the timer (drop order).
> +//
> +// By using a handle, we can make the handle own the callback target and avoid these problems.
> +
>  use super::{ClockSource, Delta, Instant};
>  use crate::{prelude::*, types::Opaque};
>  use core::{marker::PhantomData, ptr::NonNull};
> 
> ---
> base-commit: 05f7e89ab9731565d8a62e3b5d1ec206485eeb0b
> change-id: 20260215-hrtimer-docs-52ec9c020285
> 
> Best regards,
> -- 
> Andreas Hindborg <a.hindborg@kernel.org>
> 
>