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charset="utf-8" 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`. 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_*` methods 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. 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] Signed-off-by: Alice Ryhl --- rust/kernel/list.rs | 325 +++++++++++++++++++++++++++++++------------- 1 file changed, 231 insertions(+), 94 deletions(-) diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs index fb93330f4af4..328d3e369d57 100644 --- a/rust/kernel/list.rs +++ b/rust/kernel/list.rs @@ -245,8 +245,20 @@ pub fn is_empty(&self) -> bool { self.first.is_null() } =20 - /// Add the provided item to the back of the list. - pub fn push_back(&mut self, item: ListArc) { + /// Inserts `item` before `next` in the cycle. + /// + /// Returns a pointer to the newly inserted element. Never changes `se= lf.first` unless the list + /// is empty. + /// + /// # Safety + /// + /// * `next` must be an element in this list or null. + /// * if `next` is null, then the list must be empty. + unsafe fn insert_inner( + &mut self, + item: ListArc, + next: *mut ListLinksFields, + ) -> *mut ListLinksFields { let raw_item =3D ListArc::into_raw(item); // SAFETY: // * We just got `raw_item` from a `ListArc`, so it's in an `Arc`. @@ -259,16 +271,16 @@ pub fn push_back(&mut self, item: ListArc) { // SAFETY: We have not yet called `post_remove`, so `list_links` i= s still valid. let item =3D unsafe { ListLinks::fields(list_links) }; =20 - if self.first.is_null() { - self.first =3D item; + // Check if the list is empty. + if next.is_null() { // SAFETY: The caller just gave us ownership of these fields. // INVARIANT: A linked list with one item should be cyclic. unsafe { (*item).next =3D item; (*item).prev =3D item; } + self.first =3D item; } else { - let next =3D self.first; // SAFETY: By the type invariant, this pointer is valid or nul= l. We just checked that // it's not null, so it must be valid. let prev =3D unsafe { (*next).prev }; @@ -282,45 +294,24 @@ pub fn push_back(&mut self, item: ListArc) { (*next).prev =3D item; } } + + item } =20 + /// Add the provided item to the back of the list. + pub fn push_back(&mut self, item: ListArc) { + // SAFETY: + // * `self.first` is null or in the list. + // * `self.first` is only null if the list is empty. + unsafe { self.insert_inner(item, self.first) }; + } + /// Add the provided item to the front of the list. pub fn push_front(&mut self, item: ListArc) { - let raw_item =3D ListArc::into_raw(item); - // SAFETY: - // * We just got `raw_item` from a `ListArc`, so it's in an `Arc`. - // * If this requirement is violated, then the previous caller of = `prepare_to_insert` - // violated the safety requirement that they can't give up owner= ship of the `ListArc` - // until they call `post_remove`. - // * We own the `ListArc`. - // * Removing items] from this list is always done using `remove_i= nternal_inner`, which - // calls `post_remove` before giving up ownership. - let list_links =3D unsafe { T::prepare_to_insert(raw_item) }; - // SAFETY: We have not yet called `post_remove`, so `list_links` i= s still valid. - let item =3D unsafe { ListLinks::fields(list_links) }; - - if self.first.is_null() { - // SAFETY: The caller just gave us ownership of these fields. - // INVARIANT: A linked list with one item should be cyclic. - unsafe { - (*item).next =3D item; - (*item).prev =3D item; - } - } else { - let next =3D self.first; - // SAFETY: We just checked that `next` is non-null. - let prev =3D unsafe { (*next).prev }; - // SAFETY: Pointers in a linked list are never dangling, and t= he caller just gave us - // ownership of the fields on `item`. - // INVARIANT: This correctly inserts `item` between `prev` and= `next`. - unsafe { - (*item).next =3D next; - (*item).prev =3D prev; - (*prev).next =3D item; - (*next).prev =3D item; - } - } - self.first =3D item; + // SAFETY: + // * `self.first` is null or in the list. + // * `self.first` is only null if the list is empty. + self.first =3D unsafe { self.insert_inner(item, self.first) }; } =20 /// Removes the last item from this list. @@ -489,17 +480,21 @@ pub fn push_all_back(&mut self, other: &mut List) { other.first =3D ptr::null_mut(); } =20 - /// 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> { - 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, + } + } =20 @@ -579,69 +574,211 @@ fn next(&mut self) -> Option> { =20 /// A cursor into a [`List`]. /// +/// A cursor always rests between two elements in the list. This means tha= t a cursor has a previous +/// and next element, but no current element. It also means that it's poss= ible to have a cursor +/// into an empty list. +/// /// # 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, const ID: u64 =3D 0> { - current: *mut ListLinksFields, list: &'a mut List, + /// Points at the element after this cursor, or null if the cursor is = after the last element. + next: *mut ListLinksFields, } =20 impl<'a, T: ?Sized + ListItem, const ID: u64> Cursor<'a, T, ID> { + /// 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 =3D self.next; + let first =3D self.list.first; + if next =3D=3D 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 =3D 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 =3D=3D first` check. Thus, `n= ext` is an element in the + // list, so we can access its `prev` pointer. + unsafe { (*next).prev } + } + - /// 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 =3D unsafe { T::view_value(ListLinks::from_fields(self.curr= ent)) }; - // 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 lis= t would require mutable - // access to the cursor or the list. However, the `ArcBorrow` ho= lds an immutable borrow - // on the cursor, which in turn holds a mutable borrow on the li= st, so any such - // mutable access requires first releasing the immutable borrow = on the cursor. - // * 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) } - } + /// Access the element after this cursor. + pub fn peek_next(&mut self) -> Option>= { + if self.next.is_null() { + return None; + } + + // INVARIANT: + // * We just checked that `self.next` is non-null, so it must be i= n `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= > { + let prev =3D self.prev_ptr(); + + if prev.is_null() { + return None; + } + + // INVARIANT: + // * We just checked that `prev` is non-null, so it must be in `se= lf.list`. + // * `self.prev_ptr()` never returns `self.next`. + Some(CursorPeek { + ptr: prev, + cursor: self, + }) + } =20 - /// Move the cursor to the next element. - pub fn next(self) -> Option> { - // SAFETY: The `current` field is always in a list. - let next =3D unsafe { (*self.current).next }; - - if next =3D=3D 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, - }) - } - } + /// Move the cursor one element forward. + /// + /// If the cursor is after the last element, then the cursor will move= back to the beginning. + pub fn move_next(&mut self) { + if self.next.is_null() { + // INVARIANT: `list.first` is in the list or null. + self.next =3D self.list.first; + return; + } + + // 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 =3D unsafe { (*self.next).next }; + + if next =3D=3D self.list.first { + next =3D core::ptr::null_mut(); + } + + // INVARIANT: `next` is either null or the next element after an e= lement in the list. + self.next =3D next; + } =20 - /// Move the cursor to the previous element. - pub fn prev(self) -> Option> { - // SAFETY: The `current` field is always in a list. - let prev =3D unsafe { (*self.current).prev }; - - if self.current =3D=3D 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, - }) - } - } + /// Move the cursor one element backwards. + /// + /// If the cursor is before the first element, then the cursor will mo= ve to the end of the + /// list. + pub fn move_prev(&mut self) { + if self.next =3D=3D self.list.first { + // We are before the first element, so move the cursor after t= he last element. + // INVARIANT: `next` can be a null pointer. + self.next =3D core::ptr::null_mut(); + return; + } + + // INVARIANT: `prev_ptr()` always returns a pointer that is null o= r in the list. + self.next =3D self.prev_ptr(); + } + + /// Inserts an element where the cursor is pointing and get a pointer = to the new element. + fn insert_inner(&mut self, item: ListArc) -> *mut ListLinksFiel= ds { + let ptr =3D 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. + unsafe { self.list.insert_inner(item, ptr) } + } + + /// Inserts an element after this cursor. + pub fn insert_next(&mut self, item: ListArc) { + self.next =3D self.insert_inner(item); + } + + /// Inserts an element before this cursor. + pub fn insert_prev(&mut self, item: ListArc) { + self.insert_inner(item); + } =20 - /// Remove the current element from the list. - pub fn remove(self) -> ListArc { - // SAFETY: The `current` pointer always points at a member of the = list. - unsafe { self.list.remove_internal(self.current) } - } + /// Remove the next element from the list. + pub fn remove_next(&mut self) -> Option> { + self.peek_next().map(|v| v.remove()) + } + + /// Remove the previous element from the list. + pub fn remove_prev(&mut self) -> Option> { + 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 =3D=3D (self.ptr =3D=3D self.cursor.next)`. +pub struct CursorPeek<'a, 'b, T: ?Sized + ListItem, const ISNEXT: bool= , const ID: u64> { + cursor: &'a mut Cursor<'b, T, ID>, + ptr: *mut ListLinksFields, +} + +impl<'a, 'b, T: ?Sized + ListItem, const ISNEXT: bool, const ID: u64> + CursorPeek<'a, 'b, T, ISNEXT, ID> +{ + /// Remove the element from the list. + pub fn remove(self) -> ListArc { + 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, s= o `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 =3D 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 lis= t would require mutable + // access to the `CursorPeek`, the `Cursor` or the `List`. Howev= er, 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) } + } +} + +impl<'a, 'b, T: ?Sized + ListItem, const ISNEXT: bool, const ID: u64> = core::ops::Deref + for CursorPeek<'a, 'b, T, ISNEXT, ID> +{ + // This can't use `ArcBorrow<'a, T>` as the target type because 'a is = too long. It would let + // you obtain an `ArcBorrow<'a, T>` and then call `CursorPeek::remove`= without giving up the + // `ArcBorrow<'a, T>`. + type Target =3D T; + + fn deref(&self) -> &T { + // SAFETY: `self.ptr` points at an element in `self.cursor.list`. + let me =3D unsafe { T::view_value(ListLinks::from_fields(self.curs= or.next)) }; + + // SAFETY: The value cannot be removed from the list for the durat= ion of the lifetime + // annotated on the returned `&T`, because removing it from the li= st 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 mut= able borrow on the + // `Cursor`, which in turn holds a mutable borrow on the `List`, s= o any such mutable access + // requires first releasing the immutable borrow on the `CursorPee= k`. + unsafe { &*me } + } +} =20 --- base-commit: 6ce162a002657910104c7a07fb50017681bc476c change-id: 20241016-cursor-between-154bed859e27 Best regards, --=20 Alice Ryhl