Re: [PATCH 2/2] rust: workqueue: remove HasWork::OFFSET

[Benno Lossin <benno.lossin@proton.me>]

On Fri Mar 14, 2025 at 9:44 PM CET, Tamir Duberstein wrote:
> On Fri, Mar 14, 2025 at 3:20 PM Benno Lossin <benno.lossin@proton.me> wrote:
>>
>> On Fri Mar 7, 2025 at 10:58 PM CET, Tamir Duberstein wrote:
>> > Implement `HasWork::work_container_of` in `impl_has_work!`, narrowing
>> > the interface of `HasWork` and replacing pointer arithmetic with
>> > `container_of!`. Remove the provided implementation of
>> > `HasWork::get_work_offset` without replacement; an implementation is
>> > already generated in `impl_has_work!`. Remove the `Self: Sized` bound on
>> > `HasWork::work_container_of` which was apparently necessary to access
>> > `OFFSET` as `OFFSET` no longer exists.
>> >
>> > A similar API change was discussed on the hrtimer series[1].
>> >
>> > Link: https://lore.kernel.org/all/20250224-hrtimer-v3-v6-12-rc2-v9-1-5bd3bf0ce6cc@kernel.org/ [1]
>> > Signed-off-by: Tamir Duberstein <tamird@gmail.com>
>> > ---
>> >  rust/kernel/workqueue.rs | 45 ++++++++++++---------------------------------
>> >  1 file changed, 12 insertions(+), 33 deletions(-)
>>
>> What is the motivation of this change? I didn't follow the discussion,
>> so if you explained it there, it would be nice if you could also add it
>> to this commit message.
>
> The motivation is right at the top: it narrows the interface and
> replaces pointer arithmetic with an existing macro, and then deletes
> unnecessary code.
>
>> > diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
>> > index 0cd100d2aefb..0e2e0ecc58a6 100644
>> > --- a/rust/kernel/workqueue.rs
>> > +++ b/rust/kernel/workqueue.rs
>> > @@ -429,51 +429,23 @@ pub unsafe fn raw_get(ptr: *const Self) -> *mut bindings::work_struct {
>> >  ///
>> >  /// # Safety
>> >  ///
>> > -/// The [`OFFSET`] constant must be the offset of a field in `Self` of type [`Work<T, ID>`]. The
>> > -/// methods on this trait must have exactly the behavior that the definitions given below have.
>> > +/// The methods on this trait must have exactly the behavior that the definitions given below have.
>> >  ///
>> >  /// [`impl_has_work!`]: crate::impl_has_work
>> > -/// [`OFFSET`]: HasWork::OFFSET
>> >  pub unsafe trait HasWork<T, const ID: u64 = 0> {
>> > -    /// The offset of the [`Work<T, ID>`] field.
>> > -    const OFFSET: usize;
>> > -
>> > -    /// Returns the offset of the [`Work<T, ID>`] field.
>> > -    ///
>> > -    /// This method exists because the [`OFFSET`] constant cannot be accessed if the type is not
>> > -    /// [`Sized`].
>> > -    ///
>> > -    /// [`OFFSET`]: HasWork::OFFSET
>> > -    #[inline]
>> > -    fn get_work_offset(&self) -> usize {
>> > -        Self::OFFSET
>> > -    }
>> > -
>> >      /// Returns a pointer to the [`Work<T, ID>`] field.
>> >      ///
>> >      /// # Safety
>> >      ///
>> >      /// The provided pointer must point at a valid struct of type `Self`.
>> > -    #[inline]
>> > -    unsafe fn raw_get_work(ptr: *mut Self) -> *mut Work<T, ID> {
>> > -        // SAFETY: The caller promises that the pointer is valid.
>> > -        unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut Work<T, ID> }
>> > -    }
>> > +    unsafe fn raw_get_work(ptr: *mut Self) -> *mut Work<T, ID>;
>> >
>> >      /// Returns a pointer to the struct containing the [`Work<T, ID>`] field.
>> >      ///
>> >      /// # Safety
>> >      ///
>> >      /// The pointer must point at a [`Work<T, ID>`] field in a struct of type `Self`.
>> > -    #[inline]
>> > -    unsafe fn work_container_of(ptr: *mut Work<T, ID>) -> *mut Self
>> > -    where
>> > -        Self: Sized,
>>
>> This bound is required in order to allow the usage of `dyn HasWork` (ie
>> object safety), so it should stay.
>>
>> Maybe add a comment explaining why it's there.
>
> I guess a doctest would be better, but I still don't understand why
> the bound is needed. Sorry, can you cite something or explain in more
> detail please?

Here is a link: https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility

But I realized that the trait wasn't object safe to begin with due to
the `OFFSET` associated constant. So I'm not sure we need this. Alice,
do you need `dyn HasWork`?

---
Cheers,
Benno