Message-ID: <859dbf29-dafe-4c04-a0ea-d88c26fd4995@nvidia.com>
Date: Mon, 9 Feb 2026 14:41:22 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Gary Guo <gary@...yguo.net>, 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 2/6/2026 10:25 AM, Gary Guo wrote:
> 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>

Thanks! I also replied below.

> 
> 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.

Yes, I am dropping it now. As per the other discussions, I will change the
module level and all the items in the module to pub. And then we will not need
this config.

> 
>> +
>>  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.

I had to add this because a patch was missing in the DRM tree it depended on. I
believe I should not need it anymore, so I will drop it. Thank you for pointing
that out.
>> +
>> +__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.
> 

Will do, thanks.

>> +                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]

Will do.

Thanks,
-- 
Joel Fernandes

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