Re: [PATCH v3] rust: workqueue: remove HasWork::OFFSET

[Alice Ryhl]

On Fri, Apr 11, 2025 at 4:08 PM Tamir Duberstein <tamird@xxxxxxxxx> 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@xxxxxxxxxx/ [1]
> Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Tested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>

Seems reasonable enough.
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

> ---
> Changes in v3:
> - Extract first commit to its own series as it is shared with other
>   series.
> - Reword `HasWork` safety comment.
> - Link to v2: https://lore.kernel.org/r/20250409-no-offset-v2-0-dda8e141a909@xxxxxxxxx
>
> Changes in v2:
> - Rebase on v6.15-rc1.
> - Add WORKQUEUE maintainers to cc.
> - Link to v1: https://lore.kernel.org/r/20250307-no-offset-v1-0-0c728f63b69c@xxxxxxxxx
> ---
>  rust/kernel/workqueue.rs | 50 ++++++++++++++++--------------------------------
>  1 file changed, 17 insertions(+), 33 deletions(-)
>
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index f98bd02b838f..d092112d843f 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -429,51 +429,28 @@ 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 [`raw_get_work`] and [`work_container_of`] must return valid pointers and must be
> +/// true inverses of each other; that is, they must satisfy the following invariants:
> +/// - `work_container_of(raw_get_work(ptr)) == ptr` for any `ptr: *mut Self`.
> +/// - `raw_get_work(work_container_of(ptr)) == ptr` for any `ptr: *mut Work<T, ID>`.
>  ///
>  /// [`impl_has_work!`]: crate::impl_has_work
> -/// [`OFFSET`]: HasWork::OFFSET
> +/// [`raw_get_work`]: HasWork::raw_get_work
> +/// [`work_container_of`]: HasWork::work_container_of
>  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,
> -    {
> -        // SAFETY: The caller promises that the pointer points at a field of the right type in the
> -        // right kind of struct.
> -        unsafe { (ptr as *mut u8).sub(Self::OFFSET) as *mut Self }
> -    }
> +    unsafe fn work_container_of(ptr: *mut Work<T, ID>) -> *mut Self;
>  }
>
>  /// Used to safely implement the [`HasWork<T, ID>`] trait.
> @@ -504,8 +481,6 @@ macro_rules! impl_has_work {
>          // SAFETY: The implementation of `raw_get_work` only compiles if the field has the right
>          // type.
>          unsafe impl$(<$($generics)+>)? $crate::workqueue::HasWork<$work_type $(, $id)?> for $self {
> -            const OFFSET: usize = ::core::mem::offset_of!(Self, $field) as usize;
> -
>              #[inline]
>              unsafe fn raw_get_work(ptr: *mut Self) -> *mut $crate::workqueue::Work<$work_type $(, $id)?> {
>                  // SAFETY: The caller promises that the pointer is not dangling.
> @@ -513,6 +488,15 @@ unsafe fn raw_get_work(ptr: *mut Self) -> *mut $crate::workqueue::Work<$work_typ
>                      ::core::ptr::addr_of_mut!((*ptr).$field)
>                  }
>              }
> +
> +            #[inline]
> +            unsafe fn work_container_of(
> +                ptr: *mut $crate::workqueue::Work<$work_type $(, $id)?>,
> +            ) -> *mut Self {
> +                // SAFETY: The caller promises that the pointer points at a field of the right type
> +                // in the right kind of struct.
> +                unsafe { $crate::container_of!(ptr, Self, $field) }
> +            }
>          }
>      )*};
>  }
>
> ---
> base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
> change-id: 20250307-no-offset-e463667a72fb
> prerequisite-change-id: 20250409-container-of-mutness-b153dab4388d:v1
> prerequisite-patch-id: 53d5889db599267f87642bb0ae3063c29bc24863
>
> Best regards,
> --
> Tamir Duberstein <tamird@xxxxxxxxx>
>