Re: [PATCH v4 2/2] rust: hrtimer: Make HrTimer repr(transparent)

[FUJITA Tomonori <tomo@flapping.org>]

On Mon, 10 Aug 2026 15:28:06 +0200
Andreas Hindborg <a.hindborg@kernel.org> wrote:

> FUJITA Tomonori <tomo@flapping.org> writes:
> 
>> On Mon, 10 Aug 2026 13:07:20 +0200
>> Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>>> "FUJITA Tomonori" <tomo@flapping.org> writes:
>>> 
>>>> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
>>>>
>>>> HrTimerCallbackContext acquires a &HrTimer<T> from a
>>>> NonNull<HrTimer<T>> while a &mut HrTimer<T> can exist at the same
>>>> time. This is sound only because HrTimer's sole field is
>>>> Opaque<bindings::hrtimer>, which puts every byte behind an UnsafeCell.
>>>> Adding a field to HrTimer that is not Opaque would make acquiring that
>>>> shared reference unsound.
>>>>
>>>> Make HrTimer repr(transparent), which prevents multiple fields, so that
>>>> such a refactor fails to compile instead of silently introducing
>>>> unsoundness. This does not guarantee the remaining field stays behind
>>>> Opaque, but it rules out the likely way of getting there.
>>>>
>>>> repr(transparent) cannot be combined with repr(C), so drop the latter.
>>>>
>>>> Suggested-by: Miguel Ojeda <ojeda@kernel.org>
>>>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
>>>> ---
>>>>  rust/kernel/time/hrtimer.rs         | 6 +++++-
>>>>  rust/kernel/time/hrtimer/arc.rs     | 2 +-
>>>>  rust/kernel/time/hrtimer/pin.rs     | 2 +-
>>>>  rust/kernel/time/hrtimer/pin_mut.rs | 2 +-
>>>>  rust/kernel/time/hrtimer/tbox.rs    | 2 +-
>>>>  5 files changed, 9 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>>>> index 1db84cd4cbe8..04f47af3541b 100644
>>>> --- a/rust/kernel/time/hrtimer.rs
>>>> +++ b/rust/kernel/time/hrtimer.rs
>>>> @@ -418,8 +418,12 @@
>>>>  /// # Invariants
>>>>  ///
>>>>  /// * `self.timer` is initialized by `bindings::hrtimer_setup`.
>>>> +// `repr(transparent)` is not merely about layout. `HrTimerCallbackContext` acquires a
>>>> +// `&HrTimer<T>` while a `&mut HrTimer<T>` may exist, which is sound only because every byte of
>>>> +// this type sits inside `Opaque`. Being transparent rejects a second field at compile time,
>>>> +// but it does not enforce that the remaining field stays `Opaque`.
>>> 
>>> Missing bullet. With that fixed:
>>> 
>>> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
>>
>> This is a plain `//` comment, not documentation. Did you mean that you
>> want it documented as one of the `# Invariants` bullets instead?
> 
> Ah, thanks for clarifying, I did not see that. No I guess it is fine.
> Maybe add a newline?

Documentation/rust/coding-guidelines.rst gives an example where a
comment follows the documentation with no blank line in between.

The existing code follows that too, so I think we should stay
consistent with the documented convention here.