rust: workqueue: remove HasWork::OFFSET

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]
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20250411-no-offset-v3-1-c0b174640ec3@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tamird

rust/kernel/workqueue.rs

@@ -429,51 +429,28 @@ impl<T: ?Sized, const ID: u64> Work<T, ID> {
 ///
 /// # 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.
@@ -504,15 +481,22 @@ 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 $(, $id)?> {
                 // SAFETY: The caller promises that the pointer is not dangling.
                 unsafe {
                     ::core::ptr::addr_of_mut!((*ptr).$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) }
+            }
         }
     )*};
 }