[PATCH 3/9] rust: list: add struct with prev/next pointers

[Alice Ryhl <aliceryhl@google.com>]

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..76597c49fa56 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 the three methods
+/// below.
+///
+/// [`ListArc<Self>`]: ListArc
+pub unsafe trait ListItem<const ID: u64 = 0>: ListArcSafe<ID> {
+    /// Views the [`ListLinks`] for this value.
+    ///
+    /// # Guarantees
+    ///
+    /// * If there is a currently active call to `prepare_to_insert`, then this returns the same
+    ///   pointer as the one returned by the currently active call to `prepare_to_insert`.
+    /// * If there is no currently active call to `prepare_to_insert`, then 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 previous 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 previous call to `prepare_to_insert`, or
+    ///   from a call to `view_links` that happened after the previous call to `prepare_to_insert`.
+    /// * Since the previous 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`.
+    ///
+    /// The caller is free to recreate the `ListArc` after this call.
+    ///
+    /// # Safety
+    ///
+    /// The provided pointer must be the pointer returned by the previous 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(),
+            }),
+        }
+    }
+}

-- 
2.44.0.478.gd926399ef9-goog