[PATCH v3 04/10] rust: list: add struct with prev/next pointers

Alice Ryhl
2024-07-23T08:23:05Z

Define the ListLinks struct, which wraps the prev/next pointers that
will be used to insert values into a List in a future patch. Also
define the ListItem trait, which is implemented by structs that have a
ListLinks field.

The ListItem trait provides four different methods that are all
essentially container_of or the reverse of container_of. Two of them are
used before inserting/after removing an item from the list, and the two
others are used when looking at a value without changing whether it is
in a list. This distinction is introduced because it is needed for the
patch that adds support for heterogeneous lists, which are implemented
by adding a third pointer field with a fat pointer to the full struct.
When inserting into the heterogeneous list, the pointer-to-self is
updated to have the right vtable, and the container_of operation is
implemented by just returning that pointer instead of using the real
container_of operation.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/list.rs | 115 ++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 115 insertions(+)

diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
index c5caa0f6105c..63e6f6a47964 100644
--- a/rust/kernel/list.rs
+++ b/rust/kernel/list.rs
@@ -4,7 +4,122 @@

//! A linked list implementation.

+use crate::init::PinInit;
+use crate::types::Opaque;
+use core::ptr;
+
mod arc;
pub use self::arc::{
impl_list_arc_safe, AtomicListArcTracker, ListArc, ListArcSafe, TryNewListArc,
};
+
+/// Implemented by types where a [`ListArc<Self>`] can be inserted into a `List`.
+///
+/// # Safety
+///
+/// Implementers must ensure that they provide the guarantees documented on methods provided by
+/// this trait.
+///
+/// [`ListArc<Self>`]: ListArc
+pub unsafe trait ListItem<const ID: u64 = 0>: ListArcSafe<ID> {
+ /// Views the [`ListLinks`] for this value.
+ ///
+ /// # Guarantees
+ ///
+ /// If there is a previous call to `prepare_to_insert` and there is no call to `post_remove`
+ /// since the most recent such call, then this returns the same pointer as the one returned by
+ /// the most recent call to `prepare_to_insert`.
+ ///
+ /// Otherwise, the returned pointer points at a read-only [`ListLinks`] with two null pointers.
+ ///
+ /// # Safety
+ ///
+ /// The provided pointer must point at a valid value. (It need not be in an `Arc`.)
+ unsafe fn view_links(me: *const Self) -> *mut ListLinks<ID>;
+
+ /// View the full value given its [`ListLinks`] field.
+ ///
+ /// Can only be used when the value is in a list.
+ ///
+ /// # Guarantees
+ ///
+ /// * Returns the same pointer as the one passed to the most recent call to `prepare_to_insert`.
+ /// * The returned pointer is valid until the next call to `post_remove`.
+ ///
+ /// # Safety
+ ///
+ /// * The provided pointer must originate from the most recent call to `prepare_to_insert`, or
+ /// from a call to `view_links` that happened after the most recent call to
+ /// `prepare_to_insert`.
+ /// * Since the most recent call to `prepare_to_insert`, the `post_remove` method must not have
+ /// been called.
+ unsafe fn view_value(me: *mut ListLinks<ID>) -> *const Self;
+
+ /// This is called when an item is inserted into a `List`.
+ ///
+ /// # Guarantees
+ ///
+ /// The caller is granted exclusive access to the returned [`ListLinks`] until `post_remove` is
+ /// called.
+ ///
+ /// # Safety
+ ///
+ /// * The provided pointer must point at a valid value in an [`Arc`].
+ /// * Calls to `prepare_to_insert` and `post_remove` on the same value must alternate.
+ /// * The caller must own the [`ListArc`] for this value.
+ /// * The caller must not give up ownership of the [`ListArc`] unless `post_remove` has been
+ /// called after this call to `prepare_to_insert`.
+ ///
+ /// [`Arc`]: crate::sync::Arc
+ unsafe fn prepare_to_insert(me: *const Self) -> *mut ListLinks<ID>;
+
+ /// This undoes a previous call to `prepare_to_insert`.
+ ///
+ /// # Guarantees
+ ///
+ /// The returned pointer is the pointer that was originally passed to `prepare_to_insert`.
+ ///
+ /// # Safety
+ ///
+ /// The provided pointer must be the pointer returned by the most recent call to
+ /// `prepare_to_insert`.
+ unsafe fn post_remove(me: *mut ListLinks<ID>) -> *const Self;
+}
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+struct ListLinksFields {
+ next: *mut ListLinksFields,
+ prev: *mut ListLinksFields,
+}
+
+/// The prev/next pointers for an item in a linked list.
+///
+/// # Invariants
+///
+/// The fields are null if and only if this item is not in a list.
+#[repr(transparent)]
+pub struct ListLinks<const ID: u64 = 0> {
+ #[allow(dead_code)]
+ inner: Opaque<ListLinksFields>,
+}
+
+// SAFETY: The next/prev fields of a ListLinks can be moved across thread boundaries.
+unsafe impl<const ID: u64> Send for ListLinks<ID> {}
+// SAFETY: The type is opaque so immutable references to a ListLinks are useless. Therefore, it's
+// okay to have immutable access to a ListLinks from several threads at once.
+unsafe impl<const ID: u64> Sync for ListLinks<ID> {}
+
+impl<const ID: u64> ListLinks<ID> {
+ /// Creates a new initializer for this type.
+ pub fn new() -> impl PinInit<Self> {
+ // INVARIANT: Pin-init initializers can't be used on an existing `Arc`, so this value will
+ // not be constructed in an `Arc` that already has a `ListArc`.
+ ListLinks {
+ inner: Opaque::new(ListLinksFields {
+ prev: ptr::null_mut(),
+ next: ptr::null_mut(),
+ }),
+ }
+ }
+}

--
2.45.2.1089.g2a221341d9-goog