Re: [PATCH v2 3/9] rust: list: add struct with prev/next pointers

[Benno Lossin]

On 06.05.24 11:53, Alice Ryhl wrote:
> 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@xxxxxxxxxx>
> ---
>  rust/kernel/list.rs | 116 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 116 insertions(+)
> 
> diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
> index c5caa0f6105c..b5cfbb96a392 100644
> --- a/rust/kernel/list.rs
> +++ b/rust/kernel/list.rs
> @@ -4,7 +4,123 @@
> 
>  //! 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

I would not mention the number of methods, since it is difficult to
maintain and doesn't actually provide any value (it already is incorrect :)

> +/// 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 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 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.

Are there any synchronization requirements? Am I allowed to call
`prepare_to_insert` and `post_remove` on different threads without
synchronizing?

> +    /// * 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.

As I read the requirements on `prepare_to_insert`, the caller is not
required to deconstruct the `ListArc`. For example the caller is allowed
to `clone_arc()` and then `into_raw()` and then pass that pointer to
`prepare_to_insert`.
So I would just remove this sentence.

> +    ///
> +    /// # Safety
> +    ///
> +    /// The provided pointer must be the pointer returned by the previous call to

Does "most recent call" make more sense? I find previous call a bit
weird. (also in the requirements above)

---
Cheers,
Benno

> +    /// `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(),
> +            }),
> +        }
> +    }
> +}
> 
> --
> 2.45.0.rc1.225.g2a3ae87e7f-goog
>