Re: [PATCH RFC v6 01/26] rust: clist: Add support to interface with C linked lists

[Zhi Wang <zhiw@nvidia.com>]

On Tue, 20 Jan 2026 15:42:38 -0500
Joel Fernandes <joelagnelf@nvidia.com> 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@nvidia.com>
> ---

snip

> +/// Initialize a `list_head` object to point to itself.
> +///
> +/// # Safety
> +///
> +/// `list` must be a valid pointer to a `list_head` object.
> +#[inline]
> +pub unsafe fn init_list_head(list: *mut bindings::list_head) {
> +    // SAFETY: Caller guarantees `list` is a valid pointer to a
> `list_head`.
> +    unsafe {
> +        (*list).next = list;
> +        (*list).prev = list;
> +    }
> +}
> +

Might be better to have a C helper? since INIT_LIST_HEAD() has WRITE_ONCE()
for memory ordering. This one seems not equal to it.

Z.

> +/// 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 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 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 fn as_raw(&self) -> *mut bindings::list_head {
> +        self.0.get()
> +    }
> +
> +    /// Get the next [`CListHead`] in the list.
> +    #[inline]
> +    pub 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) }
> +    }
> +
> +    /// Get the previous [`CListHead`] in the list.
> +    #[inline]
> +    pub fn prev(&self) -> &Self {
> +        let raw = self.as_raw();
> +        // SAFETY:
> +        // - self.as_raw() is valid per type invariants.
> +        // - The `prev` pointer is guaranteed to be non-NULL.
> +        unsafe { Self::from_raw((*raw).prev) }
> +    }
> +
> +    /// Check if this node is linked in a list (not isolated).
> +    #[inline]
> +    pub 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 }
> +    }
> +
> +    /// Fallible pin-initializer that initializes and then calls user
> closure.
> +    ///
> +    /// Initializes the list head first, then passes `&CListHead` to
> the closure.
> +    /// This hides the raw FFI pointer from the user.
> +    pub fn try_init<E>(
> +        init_func: impl FnOnce(&CListHead) -> Result<(), E>,
> +    ) -> impl PinInit<Self, E> {
> +        // SAFETY: init_list_head initializes the list_head to point to
> itself.
> +        // After initialization, we create a reference to pass to the
> closure.
> +        unsafe {
> +            pin_init::pin_init_from_closure(move |slot: *mut Self| {
> +                init_list_head(slot.cast());
> +                // SAFETY: slot is now initialized, safe to create
> reference.
> +                init_func(&*slot)
> +            })
> +        }
> +    }
> +}
> +
> +// 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 {
> +    fn eq(&self, other: &Self) -> bool {
> +        self.as_raw() == other.as_raw()
> +    }
> +}
> +
> +impl Eq for CListHead {}
> +
> +/// Low-level iterator over `list_head` nodes.
> +///
> +/// An iterator used to iterate over a C intrusive linked list
> (`list_head`). Caller has to +/// perform conversion of returned
> [`CListHead`] to an item (using `container_of` macro or similar). +///
> +/// # Invariants
> +///
> +/// [`CListHeadIter`] is iterating over an allocated, initialized and
> valid list. +struct CListHeadIter<'a> {
> +    current_head: &'a CListHead,
> +    list_head: &'a CListHead,
> +}
> +
> +impl<'a> Iterator for CListHeadIter<'a> {
> +    type Item = &'a CListHead;
> +
> +    #[inline]
> +    fn next(&mut self) -> Option<Self::Item> {
> +        // Advance to next node.
> +        let next = self.current_head.next();
> +
> +        // Check if we've circled back to the sentinel head.
> +        if next == self.list_head {
> +            None
> +        } else {
> +            self.current_head = next;
> +            Some(self.current_head)
> +        }
> +    }
> +}
> +
> +impl<'a> FusedIterator for CListHeadIter<'a> {}
> +
> +/// A typed C linked list with a sentinel head.
> +///
> +/// A sentinel head represents the entire linked list and can be used
> for +/// iteration over items of type `T`, it is not associated with a
> specific item. +///
> +/// The const generic `OFFSET` specifies the byte offset of the
> `list_head` field within +/// the struct that `T` wraps.
> +///
> +/// # Invariants
> +///
> +/// - `head` is an allocated and valid C `list_head` structure that is
> the list's sentinel. +/// - `OFFSET` is the byte offset of the
> `list_head` field within the struct that `T` wraps. +/// - All the
> list's `list_head` nodes are allocated and have valid next/prev
> pointers. +/// - The underlying `list_head` (and entire list) is not
> modified for the lifetime `'a`. +pub struct CList<'a, T, const OFFSET:
> usize> {
> +    head: &'a CListHead,
> +    _phantom: PhantomData<&'a T>,
> +}
> +
> +impl<'a, T, const OFFSET: usize> CList<'a, T, OFFSET> {
> +    /// Create a typed [`CList`] from a raw sentinel `list_head`
> pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// - `ptr` must be a valid pointer to an allocated and initialized
> `list_head` structure
> +    ///   representing a list sentinel.
> +    /// - `ptr` must remain valid and unmodified for the lifetime `'a`.
> +    /// - The list must contain items where the `list_head` field is at
> byte offset `OFFSET`.
> +    /// - `T` must be `#[repr(transparent)]` over the C struct.
> +    #[inline]
> +    pub unsafe fn from_raw(ptr: *mut bindings::list_head) -> Self {
> +        Self {
> +            // SAFETY: Caller guarantees `ptr` is a valid, sentinel
> `list_head` object.
> +            head: unsafe { CListHead::from_raw(ptr) },
> +            _phantom: PhantomData,
> +        }
> +    }
> +
> +    /// Get the raw sentinel `list_head` pointer.
> +    #[inline]
> +    pub fn as_raw(&self) -> *mut bindings::list_head {
> +        self.head.as_raw()
> +    }
> +
> +    /// Check if the list is empty.
> +    #[inline]
> +    pub fn is_empty(&self) -> bool {
> +        let raw = self.as_raw();
> +        // SAFETY: self.as_raw() is valid per type invariants.
> +        unsafe { (*raw).next == raw }
> +    }
> +
> +    /// Create an iterator over typed items.
> +    #[inline]
> +    pub fn iter(&self) -> CListIter<'a, T, OFFSET> {
> +        CListIter {
> +            head_iter: CListHeadIter {
> +                current_head: self.head,
> +                list_head: self.head,
> +            },
> +            _phantom: PhantomData,
> +        }
> +    }
> +}
> +
> +/// High-level iterator over typed list items.
> +pub struct CListIter<'a, T, const OFFSET: usize> {
> +    head_iter: CListHeadIter<'a>,
> +    _phantom: PhantomData<&'a T>,
> +}
> +
> +impl<'a, T, const OFFSET: usize> Iterator for CListIter<'a, T, OFFSET> {
> +    type Item = &'a T;
> +
> +    fn next(&mut self) -> Option<Self::Item> {
> +        let head = self.head_iter.next()?;
> +
> +        // Convert to item using OFFSET.
> +        // SAFETY: `item_ptr` calculation from `OFFSET` (calculated
> using offset_of!)
> +        // is valid per invariants.
> +        Some(unsafe { &*head.as_raw().byte_sub(OFFSET).cast::<T>() })
> +    }
> +}
> +
> +impl<'a, T, const OFFSET: usize> FusedIterator for CListIter<'a, T,
> OFFSET> {} +
> +/// Create a C doubly-circular linked list interface [`CList`] from a
> raw `list_head` pointer. +///
> +/// This macro creates a [`CList<T, OFFSET>`] that can iterate over
> items of type `$rust_type` +/// linked via the `$field` field in the
> underlying C struct `$c_type`. +///
> +/// # Arguments
> +///
> +/// - `$head`: Raw pointer to the sentinel `list_head` object (`*mut
> bindings::list_head`). +/// - `$rust_type`: Each item's rust wrapper
> type. +/// - `$c_type`: Each item's C struct type that contains the
> embedded `list_head`. +/// - `$field`: The name of the `list_head` field
> within the C struct. +///
> +/// # Safety
> +///
> +/// The caller must ensure:
> +/// - `$head` is a valid, initialized sentinel `list_head` pointing to
> a list that remains +///   unmodified for the lifetime of the rust
> [`CList`]. +/// - The list contains items of type `$c_type` linked via
> an embedded `$field`. +/// - `$rust_type` is `#[repr(transparent)]` over
> `$c_type` or has compatible layout. +/// - The macro is called from an
> unsafe block. +///
> +/// # Examples
> +///
> +/// Refer to the examples in the [`crate::clist`] module documentation.
> +#[macro_export]
> +macro_rules! clist_create {
> +    ($head:expr, $rust_type:ty, $c_type:ty, $($field:tt).+) => {{
> +        // Compile-time check that field path is a list_head.
> +        let _: fn(*const $c_type) -> *const $crate::bindings::list_head
> =
> +            |p| ::core::ptr::addr_of!((*p).$($field).+);
> +
> +        // Calculate offset and create `CList`.
> +        const OFFSET: usize = ::core::mem::offset_of!($c_type,
> $($field).+);
> +        $crate::clist::CList::<$rust_type, OFFSET>::from_raw($head)
> +    }};
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index f812cf120042..cd7e6a1055b0 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -75,6 +75,7 @@
>  pub mod bug;
>  #[doc(hidden)]
>  pub mod build_assert;
> +pub mod clist;
>  pub mod clk;
>  #[cfg(CONFIG_CONFIGFS_FS)]
>  pub mod configfs;