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rust: list: make the cursor point between elements

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I've been using the linked list cursor for a few different things, and I
find it inconvenient to use because all of the functions have signatures
along the lines of `Self -> Option<Self>`. The root cause of these
signatures is that the cursor points *at* an element, rather than
*between* two elements.

Thus, change the cursor API to point between two elements. This is
inspired by the stdlib linked list (well, really by this guy [1]), which
also uses cursors that point between elements.

The `peek_next` method returns a helper that lets you look at and
optionally remove the element, as one common use-case of cursors is to
iterate a list to look for an element, then remove that element.

For many of the methods, this will reduce how many we need since they
now just need a prev/next method, instead of the current state where you
may end up needing all of curr/prev/next. Also, if we decide to add a
function for splitting a list into two lists at the cursor, then a
cursor that points between elements is exactly what makes the most
sense.

Another advantage is that this means you can now have a cursor into an
empty list.

Link: https://rust-unofficial.github.io/too-many-lists/sixth-cursors-intro.html [1]
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250210-cursor-between-v7-2-36f0215181ed@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Alice Ryhl 2025-02-10 09:53:36 +00:00 • committed by Miguel Ojeda
parent 998c65733b
commit 52ae96f518
1 changed files with 349 additions and 56 deletions
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405
rust/kernel/list.rs
View file

@ -483,17 +483,21 @@ impl<T: ?Sized + ListItem<ID>, const ID: u64> List<T, ID> {
other.first = ptr::null_mut();
}
/// Returns a cursor to the first element of the list.
///
/// If the list is empty, this returns `None`.
pub fn cursor_front(&mut self) -> Option<Cursor<'_, T, ID>> {
if self.first.is_null() {
None
} else {
Some(Cursor {
current: self.first,
list: self,
})
/// Returns a cursor that points before the first element of the list.
pub fn cursor_front(&mut self) -> Cursor<'_, T, ID> {
// INVARIANT: `self.first` is in this list.
Cursor {
next: self.first,
list: self,
}
}
/// Returns a cursor that points after the last element in the list.
pub fn cursor_back(&mut self) -> Cursor<'_, T, ID> {
// INVARIANT: `next` is allowed to be null.
Cursor {
next: core::ptr::null_mut(),
list: self,
}
}
@ -573,69 +577,358 @@ impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Iterator for Iter<'a, T, ID> {
/// A cursor into a [`List`].
///
/// A cursor always rests between two elements in the list. This means that a cursor has a previous
/// and next element, but no current element. It also means that it's possible to have a cursor
/// into an empty list.
///
/// # Examples
///
/// ```
/// use kernel::prelude::*;
/// use kernel::list::{List, ListArc, ListLinks};
///
/// #[pin_data]
/// struct ListItem {
/// value: u32,
/// #[pin]
/// links: ListLinks,
/// }
///
/// impl ListItem {
/// fn new(value: u32) -> Result<ListArc<Self>> {
/// ListArc::pin_init(try_pin_init!(Self {
/// value,
/// links <- ListLinks::new(),
/// }), GFP_KERNEL)
/// }
/// }
///
/// kernel::list::impl_has_list_links! {
/// impl HasListLinks<0> for ListItem { self.links }
/// }
/// kernel::list::impl_list_arc_safe! {
/// impl ListArcSafe<0> for ListItem { untracked; }
/// }
/// kernel::list::impl_list_item! {
/// impl ListItem<0> for ListItem { using ListLinks; }
/// }
///
/// // Use a cursor to remove the first element with the given value.
/// fn remove_first(list: &mut List<ListItem>, value: u32) -> Option<ListArc<ListItem>> {
/// let mut cursor = list.cursor_front();
/// while let Some(next) = cursor.peek_next() {
/// if next.value == value {
/// return Some(next.remove());
/// }
/// cursor.move_next();
/// }
/// None
/// }
///
/// // Use a cursor to remove the last element with the given value.
/// fn remove_last(list: &mut List<ListItem>, value: u32) -> Option<ListArc<ListItem>> {
/// let mut cursor = list.cursor_back();
/// while let Some(prev) = cursor.peek_prev() {
/// if prev.value == value {
/// return Some(prev.remove());
/// }
/// cursor.move_prev();
/// }
/// None
/// }
///
/// // Use a cursor to remove all elements with the given value. The removed elements are moved to
/// // a new list.
/// fn remove_all(list: &mut List<ListItem>, value: u32) -> List<ListItem> {
/// let mut out = List::new();
/// let mut cursor = list.cursor_front();
/// while let Some(next) = cursor.peek_next() {
/// if next.value == value {
/// out.push_back(next.remove());
/// } else {
/// cursor.move_next();
/// }
/// }
/// out
/// }
///
/// // Use a cursor to insert a value at a specific index. Returns an error if the index is out of
/// // bounds.
/// fn insert_at(list: &mut List<ListItem>, new: ListArc<ListItem>, idx: usize) -> Result {
/// let mut cursor = list.cursor_front();
/// for _ in 0..idx {
/// if !cursor.move_next() {
/// return Err(EINVAL);
/// }
/// }
/// cursor.insert_next(new);
/// Ok(())
/// }
///
/// // Merge two sorted lists into a single sorted list.
/// fn merge_sorted(list: &mut List<ListItem>, merge: List<ListItem>) {
/// let mut cursor = list.cursor_front();
/// for to_insert in merge {
/// while let Some(next) = cursor.peek_next() {
/// if to_insert.value < next.value {
/// break;
/// }
/// cursor.move_next();
/// }
/// cursor.insert_prev(to_insert);
/// }
/// }
///
/// let mut list = List::new();
/// list.push_back(ListItem::new(14)?);
/// list.push_back(ListItem::new(12)?);
/// list.push_back(ListItem::new(10)?);
/// list.push_back(ListItem::new(12)?);
/// list.push_back(ListItem::new(15)?);
/// list.push_back(ListItem::new(14)?);
/// assert_eq!(remove_all(&mut list, 12).iter().count(), 2);
/// // [14, 10, 15, 14]
/// assert!(remove_first(&mut list, 14).is_some());
/// // [10, 15, 14]
/// insert_at(&mut list, ListItem::new(12)?, 2)?;
/// // [10, 15, 12, 14]
/// assert!(remove_last(&mut list, 15).is_some());
/// // [10, 12, 14]
///
/// let mut list2 = List::new();
/// list2.push_back(ListItem::new(11)?);
/// list2.push_back(ListItem::new(13)?);
/// merge_sorted(&mut list, list2);
///
/// let mut items = list.into_iter();
/// assert_eq!(items.next().unwrap().value, 10);
/// assert_eq!(items.next().unwrap().value, 11);
/// assert_eq!(items.next().unwrap().value, 12);
/// assert_eq!(items.next().unwrap().value, 13);
/// assert_eq!(items.next().unwrap().value, 14);
/// assert!(items.next().is_none());
/// # Result::<(), Error>::Ok(())
/// ```
///
/// # Invariants
///
/// The `current` pointer points a value in `list`.
/// The `next` pointer is null or points a value in `list`.
pub struct Cursor<'a, T: ?Sized + ListItem<ID>, const ID: u64 = 0> {
current: *mut ListLinksFields,
list: &'a mut List<T, ID>,
/// Points at the element after this cursor, or null if the cursor is after the last element.
next: *mut ListLinksFields,
}
impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Cursor<'a, T, ID> {
/// Access the current element of this cursor.
pub fn current(&self) -> ArcBorrow<'_, T> {
// SAFETY: The `current` pointer points a value in the list.
let me = unsafe { T::view_value(ListLinks::from_fields(self.current)) };
/// Returns a pointer to the element before the cursor.
///
/// Returns null if there is no element before the cursor.
fn prev_ptr(&self) -> *mut ListLinksFields {
let mut next = self.next;
let first = self.list.first;
if next == first {
// We are before the first element.
return core::ptr::null_mut();
}
if next.is_null() {
// We are after the last element, so we need a pointer to the last element, which is
// the same as `(*first).prev`.
next = first;
}
// SAFETY: `next` can't be null, because then `first` must also be null, but in that case
// we would have exited at the `next == first` check. Thus, `next` is an element in the
// list, so we can access its `prev` pointer.
unsafe { (*next).prev }
}
/// Access the element after this cursor.
pub fn peek_next(&mut self) -> Option<CursorPeek<'_, 'a, T, true, ID>> {
if self.next.is_null() {
return None;
}
// INVARIANT:
// * We just checked that `self.next` is non-null, so it must be in `self.list`.
// * `ptr` is equal to `self.next`.
Some(CursorPeek {
ptr: self.next,
cursor: self,
})
}
/// Access the element before this cursor.
pub fn peek_prev(&mut self) -> Option<CursorPeek<'_, 'a, T, false, ID>> {
let prev = self.prev_ptr();
if prev.is_null() {
return None;
}
// INVARIANT:
// * We just checked that `prev` is non-null, so it must be in `self.list`.
// * `self.prev_ptr()` never returns `self.next`.
Some(CursorPeek {
ptr: prev,
cursor: self,
})
}
/// Move the cursor one element forward.
///
/// If the cursor is after the last element, then this call does nothing. This call returns
/// `true` if the cursor's position was changed.
pub fn move_next(&mut self) -> bool {
if self.next.is_null() {
return false;
}
// SAFETY: `self.next` is an element in the list and we borrow the list mutably, so we can
// access the `next` field.
let mut next = unsafe { (*self.next).next };
if next == self.list.first {
next = core::ptr::null_mut();
}
// INVARIANT: `next` is either null or the next element after an element in the list.
self.next = next;
true
}
/// Move the cursor one element backwards.
///
/// If the cursor is before the first element, then this call does nothing. This call returns
/// `true` if the cursor's position was changed.
pub fn move_prev(&mut self) -> bool {
if self.next == self.list.first {
return false;
}
// INVARIANT: `prev_ptr()` always returns a pointer that is null or in the list.
self.next = self.prev_ptr();
true
}
/// Inserts an element where the cursor is pointing and get a pointer to the new element.
fn insert_inner(&mut self, item: ListArc<T, ID>) -> *mut ListLinksFields {
let ptr = if self.next.is_null() {
self.list.first
} else {
self.next
};
// SAFETY:
// * `ptr` is an element in the list or null.
// * if `ptr` is null, then `self.list.first` is null so the list is empty.
let item = unsafe { self.list.insert_inner(item, ptr) };
if self.next == self.list.first {
// INVARIANT: We just inserted `item`, so it's a member of list.
self.list.first = item;
}
item
}
/// Insert an element at this cursor's location.
pub fn insert(mut self, item: ListArc<T, ID>) {
// This is identical to `insert_prev`, but consumes the cursor. This is helpful because it
// reduces confusion when the last operation on the cursor is an insertion; in that case,
// you just want to insert the element at the cursor, and it is confusing that the call
// involves the word prev or next.
self.insert_inner(item);
}
/// Inserts an element after this cursor.
///
/// After insertion, the new element will be after the cursor.
pub fn insert_next(&mut self, item: ListArc<T, ID>) {
self.next = self.insert_inner(item);
}
/// Inserts an element before this cursor.
///
/// After insertion, the new element will be before the cursor.
pub fn insert_prev(&mut self, item: ListArc<T, ID>) {
self.insert_inner(item);
}
/// Remove the next element from the list.
pub fn remove_next(&mut self) -> Option<ListArc<T, ID>> {
self.peek_next().map(|v| v.remove())
}
/// Remove the previous element from the list.
pub fn remove_prev(&mut self) -> Option<ListArc<T, ID>> {
self.peek_prev().map(|v| v.remove())
}
}
/// References the element in the list next to the cursor.
///
/// # Invariants
///
/// * `ptr` is an element in `self.cursor.list`.
/// * `ISNEXT == (self.ptr == self.cursor.next)`.
pub struct CursorPeek<'a, 'b, T: ?Sized + ListItem<ID>, const ISNEXT: bool, const ID: u64> {
cursor: &'a mut Cursor<'b, T, ID>,
ptr: *mut ListLinksFields,
}
impl<'a, 'b, T: ?Sized + ListItem<ID>, const ISNEXT: bool, const ID: u64>
CursorPeek<'a, 'b, T, ISNEXT, ID>
{
/// Remove the element from the list.
pub fn remove(self) -> ListArc<T, ID> {
if ISNEXT {
self.cursor.move_next();
}
// INVARIANT: `self.ptr` is not equal to `self.cursor.next` due to the above `move_next`
// call.
// SAFETY: By the type invariants of `Self`, `next` is not null, so `next` is an element of
// `self.cursor.list` by the type invariants of `Cursor`.
unsafe { self.cursor.list.remove_internal(self.ptr) }
}
/// Access this value as an [`ArcBorrow`].
pub fn arc(&self) -> ArcBorrow<'_, T> {
// SAFETY: `self.ptr` points at an element in `self.cursor.list`.
let me = unsafe { T::view_value(ListLinks::from_fields(self.ptr)) };
// SAFETY:
// * All values in a list are stored in an `Arc`.
// * The value cannot be removed from the list for the duration of the lifetime annotated
// on the returned `ArcBorrow`, because removing it from the list would require mutable
// access to the cursor or the list. However, the `ArcBorrow` holds an immutable borrow
// on the cursor, which in turn holds a mutable borrow on the list, so any such
// mutable access requires first releasing the immutable borrow on the cursor.
// access to the `CursorPeek`, the `Cursor` or the `List`. However, the `ArcBorrow` holds
// an immutable borrow on the `CursorPeek`, which in turn holds a mutable borrow on the
// `Cursor`, which in turn holds a mutable borrow on the `List`, so any such mutable
// access requires first releasing the immutable borrow on the `CursorPeek`.
// * Values in a list never have a `UniqueArc` reference, because the list has a `ListArc`
// reference, and `UniqueArc` references must be unique.
unsafe { ArcBorrow::from_raw(me) }
}
}
/// Move the cursor to the next element.
pub fn next(self) -> Option<Cursor<'a, T, ID>> {
// SAFETY: The `current` field is always in a list.
let next = unsafe { (*self.current).next };
impl<'a, 'b, T: ?Sized + ListItem<ID>, const ISNEXT: bool, const ID: u64> core::ops::Deref
for CursorPeek<'a, 'b, T, ISNEXT, ID>
{
// If you change the `ptr` field to have type `ArcBorrow<'a, T>`, it might seem like you could
// get rid of the `CursorPeek::arc` method and change the deref target to `ArcBorrow<'a, T>`.
// However, that doesn't work because 'a is too long. You could obtain an `ArcBorrow<'a, T>`
// and then call `CursorPeek::remove` without giving up the `ArcBorrow<'a, T>`, which would be
// unsound.
type Target = T;
if next == self.list.first {
None
} else {
// INVARIANT: Since `self.current` is in the `list`, its `next` pointer is also in the
// `list`.
Some(Cursor {
current: next,
list: self.list,
})
}
}
fn deref(&self) -> &T {
// SAFETY: `self.ptr` points at an element in `self.cursor.list`.
let me = unsafe { T::view_value(ListLinks::from_fields(self.ptr)) };
/// Move the cursor to the previous element.
pub fn prev(self) -> Option<Cursor<'a, T, ID>> {
// SAFETY: The `current` field is always in a list.
let prev = unsafe { (*self.current).prev };
if self.current == self.list.first {
None
} else {
// INVARIANT: Since `self.current` is in the `list`, its `prev` pointer is also in the
// `list`.
Some(Cursor {
current: prev,
list: self.list,
})
}
}
/// Remove the current element from the list.
pub fn remove(self) -> ListArc<T, ID> {
// SAFETY: The `current` pointer always points at a member of the list.
unsafe { self.list.remove_internal(self.current) }
// SAFETY: The value cannot be removed from the list for the duration of the lifetime
// annotated on the returned `&T`, because removing it from the list would require mutable
// access to the `CursorPeek`, the `Cursor` or the `List`. However, the `&T` holds an
// immutable borrow on the `CursorPeek`, which in turn holds a mutable borrow on the
// `Cursor`, which in turn holds a mutable borrow on the `List`, so any such mutable access
// requires first releasing the immutable borrow on the `CursorPeek`.
unsafe { &*me }
}
}