linux-kernel - Re: [PATCH 8/9] rust: list: support heterogeneous lists

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Date: Thu, 04 Apr 2024 15:35:51 +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>, Kees Cook <keescook@...omium.org>, 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
Subject: Re: [PATCH 8/9] rust: list: support heterogeneous lists

On 02.04.24 14:17, Alice Ryhl wrote:
> @@ -180,6 +184,41 @@ unsafe fn from_fields(me: *mut ListLinksFields) -> *mut Self {
>      }
>  }
> 
> +/// Similar to [`ListLinks`], but also contains a pointer to the full value.
> +///
> +/// This type can be used instead of [`ListLinks`] to support lists with trait objects.
> +#[repr(C)]
> +pub struct ListLinksSelfPtr<T: ?Sized, const ID: u64 = 0> {
> +    /// The `ListLinks` field inside this value.
> +    ///
> +    /// This is public so that it can be used with `impl_has_list_links!`.
> +    pub inner: ListLinks<ID>,
> +    self_ptr: UnsafeCell<MaybeUninit<*const T>>,
> +}
> +
> +unsafe impl<T: ?Sized + Send, const ID: u64> Send for ListLinksSelfPtr<T, ID> {}
> +unsafe impl<T: ?Sized + Sync, const ID: u64> Sync for ListLinksSelfPtr<T, ID> {}

Missing SAFETY comments.

> +
> +impl<T: ?Sized, const ID: u64> ListLinksSelfPtr<T, ID> {
> +    /// The offset from the [`ListLinks`] to the self pointer field.
> +    pub const LIST_LINKS_SELF_PTR_OFFSET: usize = core::mem::offset_of!(Self, self_ptr);
> +
> +    /// 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`.
> +        Self {
> +            inner: ListLinks {
> +                inner: Opaque::new(ListLinksFields {
> +                    prev: ptr::null_mut(),
> +                    next: ptr::null_mut(),
> +                }),
> +            },

Why don't you use `inner <- ListLinks::new(),`?

> +            self_ptr: UnsafeCell::new(MaybeUninit::zeroed()),
> +        }
> +    }
> +}
> +
>  impl<T: ?Sized + ListItem<ID>, const ID: u64> List<T, ID> {
>      /// Creates a new empty list.
>      pub const fn new() -> Self {

[...]

> @@ -94,5 +137,45 @@ unsafe fn post_remove(me: *mut ListLinks<$num>) -> *const Self {
>              }
>          }
>      };
> +
> +    (
> +        impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
> +            using ListLinksSelfPtr;
> +        } $($rest:tt)*
> +    ) => {
> +        unsafe impl$(<$($generics)*>)? ListItem<$num> for $t {
> +            unsafe fn prepare_to_insert(me: *const Self) -> *mut ListLinks<$num> {
> +                let links_field = unsafe { Self::view_links(me) };
> +
> +                let spoff = ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
> +                let self_ptr = unsafe { (links_field as *const u8).add(spoff)
> +                    as *const ::core::cell::UnsafeCell<*const Self> };
> +                let cell_inner = ::core::cell::UnsafeCell::raw_get(self_ptr);
> +
> +                unsafe { ::core::ptr::write(cell_inner, me) };
> +                links_field
> +            }
> +
> +            unsafe fn view_links(me: *const Self) -> *mut ListLinks<$num> {
> +                unsafe {
> +                    <Self as HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
> +                }
> +            }
> +
> +            unsafe fn view_value(links_field: *mut ListLinks<$num>) -> *const Self {
> +                let spoff = ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
> +                let self_ptr = unsafe { (links_field as *const u8).add(spoff)
> +                    as *const ::core::cell::UnsafeCell<*const Self> };
> +                let cell_inner = ::core::cell::UnsafeCell::raw_get(self_ptr);
> +                unsafe {
> +                    ::core::ptr::read(cell_inner)
> +                }
> +            }
> +
> +            unsafe fn post_remove(me: *mut ListLinks<$num>) -> *const Self {
> +                unsafe { Self::view_value(me) }
> +            }
> +        }
> +    };

The paths in this macro should use `$crate::...` to prevent import
errors.

-- 
Cheers,
Benno