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

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Message-ID: <1b2078d8-d93b-4626-a73f-edc5616a2357@proton.me>
Date: Wed, 31 Jul 2024 18:41:38 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Marco Elver <elver@...gle.com>, Coly Li <colyli@...e.de>, Paolo Abeni <pabeni@...hat.com>, Pierre Gondois <pierre.gondois@....com>, Ingo Molnar <mingo@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Wei Yang <richard.weiyang@...il.com>, Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 04/10] rust: list: add struct with prev/next pointers

On 23.07.24 10:22, Alice Ryhl wrote:
> +/// 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>,

Do you really need `Opaque`? Or would `UnsafeCell` be enough? (If it is
enough and you change this, be aware that `Opaque` is `!Unpin`, so if
you intend for `ListLinks` to also be `!Unpin`, then you need a
`PhantomPinned`)

> +}
> +
> +// SAFETY: The next/prev fields of a ListLinks can be moved across thread boundaries.

Why? This is not a justification.

> +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.

You don't need to argue via `Opaque`, the type doesn't expose any
`&self` functions, so there are no functions to consider.

---
Cheers,
Benno

> +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
>