Message-Id: <DG7ZF1UT98RQ.3F42J3ULGV2OC@garyguo.net>
Date: Fri, 06 Feb 2026 15:25:23 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Joel Fernandes" <joelagnelf@...dia.com>, <linux-kernel@...r.kernel.org>
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Subject: Re: [PATCH -next v7 1/2] rust: clist: Add support to interface with
 C linked lists

On Fri Feb 6, 2026 at 12:41 AM GMT, Joel Fernandes wrote:
> Add a new module `clist` 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.
>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
>  MAINTAINERS            |   7 +
>  drivers/gpu/Kconfig    |   7 +
>  rust/helpers/helpers.c |   1 +
>  rust/helpers/list.c    |  21 +++
>  rust/kernel/clist.rs   | 315 +++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs     |   2 +
>  6 files changed, 353 insertions(+)
>  create mode 100644 rust/helpers/list.c
>  create mode 100644 rust/kernel/clist.rs
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 900fc00b73e6..310bb479260c 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -23204,6 +23204,13 @@ S:	Maintained
>  T:	git https://github.com/Rust-for-Linux/linux.git rust-analyzer-next
>  F:	scripts/generate_rust_analyzer.py
>  
> +RUST TO C LIST INTERFACES
> +M:	Joel Fernandes <joelagnelf@...dia.com>
> +M:	Alexandre Courbot <acourbot@...dia.com>
> +L:	rust-for-linux@...r.kernel.org
> +S:	Maintained
> +F:	rust/kernel/clist.rs

I still think we should try to work on a more powerful list infra that works for
both C and Rust, but I reckon that is a longer term effort, and shouldn't
prevent a simpler version from getting in and be used by abstractions that need
it. So

Acked-by: Gary Guo <gary@...yguo.net>

Some nits below:

> +
>  RXRPC SOCKETS (AF_RXRPC)
>  M:	David Howells <dhowells@...hat.com>
>  M:	Marc Dionne <marc.dionne@...istor.com>
> diff --git a/drivers/gpu/Kconfig b/drivers/gpu/Kconfig
> index 22dd29cd50b5..2c3dec070645 100644
> --- a/drivers/gpu/Kconfig
> +++ b/drivers/gpu/Kconfig
> @@ -1,7 +1,14 @@
>  # SPDX-License-Identifier: GPL-2.0
>  
> +config RUST_CLIST
> +	bool
> +	depends on RUST
> +	help
> +	  Rust abstraction for interfacing with C linked lists.

I am not sure if we need extra config entry. This is fully generic so shouldn't
generate any code unless there is an user.

> +
>  config GPU_BUDDY
>  	bool
> +	select RUST_CLIST if RUST
>  	help
>  	  A page based buddy allocator for GPU memory.
>  
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index a3c42e51f00a..724fcb8240ac 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -35,6 +35,7 @@
>  #include "io.c"
>  #include "jump_label.c"
>  #include "kunit.c"
> +#include "list.c"
>  #include "maple_tree.c"
>  #include "mm.c"
>  #include "mutex.c"
> diff --git a/rust/helpers/list.c b/rust/helpers/list.c
> new file mode 100644
> index 000000000000..3390b154fa36
> --- /dev/null
> +++ b/rust/helpers/list.c
> @@ -0,0 +1,21 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Helpers for C Circular doubly linked list implementation.
> + */
> +
> +#include <linux/list.h>
> +
> +#ifndef __rust_helper
> +#define __rust_helper
> +#endif

This shouldn't be needed.

> +
> +__rust_helper void rust_helper_INIT_LIST_HEAD(struct list_head *list)
> +{
> +	INIT_LIST_HEAD(list);
> +}
> +
> +__rust_helper void rust_helper_list_add_tail(struct list_head *new, struct list_head *head)
> +{
> +	list_add_tail(new, head);
> +}
> diff --git a/rust/kernel/clist.rs b/rust/kernel/clist.rs
> new file mode 100644
> index 000000000000..1f6d4db13c1d
> --- /dev/null
> +++ b/rust/kernel/clist.rs
> @@ -0,0 +1,315 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! A C doubly circular intrusive linked list interface for rust code.
> +//!
> +//! # Examples
> +//!
> +//! ```
> +//! use kernel::{
> +//!     bindings,
> +//!     clist_create,
> +//!     types::Opaque, //
> +//! };
> +//! # // Create test list with values (0, 10, 20) - normally done by C code but it is
> +//! # // emulated here for doctests using the C bindings.
> +//! # use core::mem::MaybeUninit;
> +//! #
> +//! # /// C struct with embedded `list_head` (typically will be allocated by C code).
> +//! # #[repr(C)]
> +//! # pub(crate) struct SampleItemC {
> +//! #     pub value: i32,
> +//! #     pub link: bindings::list_head,
> +//! # }
> +//! #
> +//! # let mut head = MaybeUninit::<bindings::list_head>::uninit();
> +//! #
> +//! # let head = head.as_mut_ptr();
> +//! # // SAFETY: head and all the items are test objects allocated in this scope.
> +//! # unsafe { bindings::INIT_LIST_HEAD(head) };
> +//! #
> +//! # let mut items = [
> +//! #     MaybeUninit::<SampleItemC>::uninit(),
> +//! #     MaybeUninit::<SampleItemC>::uninit(),
> +//! #     MaybeUninit::<SampleItemC>::uninit(),
> +//! # ];
> +//! #
> +//! # for (i, item) in items.iter_mut().enumerate() {
> +//! #     let ptr = item.as_mut_ptr();
> +//! #     // SAFETY: pointers are to allocated test objects with a list_head field.
> +//! #     unsafe {
> +//! #         (*ptr).value = i as i32 * 10;
> +//! #         // addr_of_mut!() computes address of link directly as link is uninitialized.
> +//! #         bindings::INIT_LIST_HEAD(core::ptr::addr_of_mut!((*ptr).link));
> +//! #         bindings::list_add_tail(&mut (*ptr).link, head);
> +//! #     }
> +//! # }
> +//!
> +//! // Rust wrapper for the C struct.
> +//! // The list item struct in this example is defined in C code as:
> +//! //   struct SampleItemC {
> +//! //       int value;
> +//! //       struct list_head link;
> +//! //   };
> +//! //
> +//! #[repr(transparent)]
> +//! pub(crate) struct Item(Opaque<SampleItemC>);
> +//!
> +//! impl Item {
> +//!     pub(crate) fn value(&self) -> i32 {
> +//!         // SAFETY: [`Item`] has same layout as [`SampleItemC`].
> +//!         unsafe { (*self.0.get()).value }
> +//!     }
> +//! }
> +//!
> +//! // Create typed [`CList`] from sentinel head.
> +//! // SAFETY: head is valid, items are [`SampleItemC`] with embedded `link` field.
> +//! let list = unsafe { clist_create!(head, Item, SampleItemC, link) };
> +//!
> +//! // Iterate directly over typed items.
> +//! let mut found_0 = false;
> +//! let mut found_10 = false;
> +//! let mut found_20 = false;
> +//!
> +//! for item in list.iter() {
> +//!     let val = item.value();
> +//!     if val == 0 { found_0 = true; }
> +//!     if val == 10 { found_10 = true; }
> +//!     if val == 20 { found_20 = true; }
> +//! }
> +//!
> +//! assert!(found_0 && found_10 && found_20);
> +//! ```
> +
> +use core::{
> +    iter::FusedIterator,
> +    marker::PhantomData, //
> +};
> +
> +use crate::{
> +    bindings,
> +    types::Opaque, //
> +};
> +
> +use pin_init::PinInit;
> +
> +/// Wraps a `list_head` object for use in intrusive linked lists.
> +///
> +/// # Invariants
> +///
> +/// - [`CListHead`] represents an allocated and valid `list_head` structure.
> +/// - Once a [`CListHead`] is created in Rust, it will not be modified by non-Rust code.
> +/// - All `list_head` for individual items are not modified for the lifetime of [`CListHead`].
> +#[repr(transparent)]
> +pub(crate) struct CListHead(Opaque<bindings::list_head>);
> +
> +impl CListHead {
> +    /// Create a `&CListHead` reference from a raw `list_head` pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// - `ptr` must be a valid pointer to an allocated and initialized `list_head` structure.
> +    /// - `ptr` must remain valid and unmodified for the lifetime `'a`.
> +    #[inline]
> +    pub(crate) unsafe fn from_raw<'a>(ptr: *mut bindings::list_head) -> &'a Self {
> +        // SAFETY:
> +        // - [`CListHead`] has same layout as `list_head`.
> +        // - `ptr` is valid and unmodified for 'a.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    /// Get the raw `list_head` pointer.
> +    #[inline]
> +    pub(crate) fn as_raw(&self) -> *mut bindings::list_head {
> +        self.0.get()
> +    }
> +
> +    /// Get the next [`CListHead`] in the list.
> +    #[inline]
> +    pub(crate) fn next(&self) -> &Self {
> +        let raw = self.as_raw();
> +        // SAFETY:
> +        // - `self.as_raw()` is valid per type invariants.
> +        // - The `next` pointer is guaranteed to be non-NULL.
> +        unsafe { Self::from_raw((*raw).next) }
> +    }
> +
> +    /// Check if this node is linked in a list (not isolated).
> +    #[inline]
> +    pub(crate) fn is_linked(&self) -> bool {
> +        let raw = self.as_raw();
> +        // SAFETY: self.as_raw() is valid per type invariants.
> +        unsafe { (*raw).next != raw && (*raw).prev != raw }
> +    }
> +
> +    /// Pin-initializer that initializes the list head.
> +    pub(crate) fn new() -> impl PinInit<Self> {
> +        // SAFETY: `INIT_LIST_HEAD` initializes `slot` to a valid empty list.
> +        unsafe {
> +            pin_init::pin_init_from_closure(move |slot: *mut Self| {

pin_init::ffi_init should be used for this.

> +                bindings::INIT_LIST_HEAD(slot.cast());
> +                Ok(())
> +            })
> +        }
> +    }
> +}
> +
> +// SAFETY: [`CListHead`] can be sent to any thread.
> +unsafe impl Send for CListHead {}
> +
> +// SAFETY: [`CListHead`] can be shared among threads as it is not modified
> +// by non-Rust code per type invariants.
> +unsafe impl Sync for CListHead {}
> +
> +impl PartialEq for CListHead {

#[inline]

> +    fn eq(&self, other: &Self) -> bool {
> +        core::ptr::eq(self, other)
> +    }
> +}
> +
> +impl Eq for CListHead {}

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