Re: [PATCH v13 1/1] rust: interop: Add list module for C linked list interface

On 3/18/2026 5:10 AM, Alice Ryhl wrote:

On Tue, Mar 17, 2026 at 04:17:10PM -0400, Joel Fernandes wrote:

quoted

Add a new module `kernel::interop::list` for working with C's doubly
circular linked lists. Provide low-level iteration over list nodes.

Typed iteration over actual items is provided with a `clist_create`
macro to assist in creation of the `CList` type.

Cc: Nikola Djukic <redacted>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Gary Guo <gary@garyguo.net>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

I have a few nits below. But overall I think this looks ok:

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Please do consider my mod.rs suggestion too, though.

sure.

quoted

+//! ```
+//! use kernel::{
+//!     bindings,
+//!     clist_create,

IMO the automatic re-exports of macros at the root shouldn't be used.
Import it from kernel::interop::list::clist_create instead.

Note that you need to put a re-export below macro definition to do this.

	macro_rules! clist_create {
	    (unsafe { $head:ident, $rust_type:ty, $c_type:ty, $($field:tt).+ }) => {{
	        // Compile-time check that field path is a `list_head`.
	        // SAFETY: `p` is a valid pointer to `$c_type`.
	        let _: fn(*const $c_type) -> *const $crate::bindings::list_head =
	            |p| unsafe { &raw const (*p).$($field).+ };
	
	        // Calculate offset and create `CList`.
	        const OFFSET: usize = ::core::mem::offset_of!($c_type, $($field).+);
	        // SAFETY: The caller of this macro is responsible for ensuring safety.
	        unsafe { $crate::interop::list::CList::<$rust_type, OFFSET>::from_raw($head) }
	    }};
	}
	pub use clist_create; // <-- you need this

See tracepoint.rs or any of the other macros for an example.

Ok.

quoted

+//! // Create typed [`CList`] from sentinel head.
+//! // SAFETY: `head` is valid and initialized, items are `SampleItemC` with
+//! // embedded `link` field, and `Item` is `#[repr(transparent)]` over `SampleItemC`.
+//! let list = clist_create!(unsafe { head, Item, SampleItemC, link });

Did you try using this in your real use-case? You require `head` to be
an :ident, but I think for any 'struct list_head' not stored on the
stack, accepting an :expr would be easier to use so that you can just
pass `&raw mut my_c_struct.the_list_head` directly to the macro. Right
now you have to put the raw pointer in a local variable first.

For buddy.rs usecase, currently we do put it in a local variable so it
works fine. Not doing :ident was causing clippy errors.


warning: this macro expands metavariables in an unsafe block
   --> rust/kernel/interop/list.rs:341:9
    |
341 |         unsafe { $crate::interop::list::CList::<$rust_type,
OFFSET>::from_raw($head) }
    |
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this allows the user of the macro to write unsafe code outside
of an unsafe block
    = help: consider expanding any metavariables outside of this block,
e.g. by storing them in a variable
    = help: ... or also expand referenced metavariables in a safe context
to require an unsafe block at callsite
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#macro_metavars_in_unsafe
    = note: `-W clippy::macro-metavars-in-unsafe` implied by `-W clippy::all`
    = help: to override `-W clippy::all` add
`#[allow(clippy::macro_metavars_in_unsafe)]`


thanks,

--
Joel Fernandes