linux-kernel - Re: [PATCH v2 5/5] rust: types: implement `ForeignOwnable` for `Arc<T>`

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Date:   Wed, 01 Feb 2023 11:17:16 +0100
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Vincenzo Palazzo <vincenzopalazzodev@...il.com>,
        Alice Ferrazzi <alice.ferrazzi@...aclelinux.com>,
        Andreas Hindborg <a.hindborg@...sung.com>
Subject: Re: [PATCH v2 5/5] rust: types: implement `ForeignOwnable` for
 `Arc<T>`


Wedson Almeida Filho <wedsonaf@...il.com> writes:

> This allows us to hand ownership of Rust ref-counted objects to
> the C side of the kernel.
>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@...il.com>
> Reviewed-by: Gary Guo <gary@...yguo.net>
> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@...il.com>
> Reviewed-by: Alice Ferrazzi <alice.ferrazzi@...aclelinux.com>
> ---
> v1 -> v2: Unmodified
>
>  rust/kernel/sync/arc.rs | 32 +++++++++++++++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index ff73f9240ca1..519a6ec43644 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -15,7 +15,11 @@
>  //!
>  //! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
>  
> -use crate::{bindings, error::Result, types::Opaque};
> +use crate::{
> +    bindings,
> +    error::Result,
> +    types::{ForeignOwnable, Opaque},
> +};
>  use alloc::boxed::Box;
>  use core::{
>      marker::{PhantomData, Unsize},
> @@ -189,6 +193,32 @@ impl<T: ?Sized> Arc<T> {
>      }
>  }
>  
> +impl<T: 'static> ForeignOwnable for Arc<T> {
> +    type Borrowed<'a> = ArcBorrow<'a, T>;
> +
> +    fn into_foreign(self) -> *const core::ffi::c_void {
> +        ManuallyDrop::new(self).ptr.as_ptr() as _
> +    }
> +
> +    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
> +        // SAFETY: By the safety requirement of this function, we know that `ptr` came from
> +        // a previous call to `Arc::into_foreign`.
> +        let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
> +
> +        // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
> +        // for the lifetime of the returned value. Additionally, the safety requirements of
> +        // `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
> +        unsafe { ArcBorrow::new(inner) }
> +    }
> +
> +    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
> +        // SAFETY: By the safety requirement of this function, we know that `ptr` came from
> +        // a previous call to `Arc::into_foreign`, which owned guarantees that `ptr` is valid and
> +        // owns a reference.

The last part of the sentence does not read clearly to me. Would this
make sense instead:

// SAFETY: By the safety requirement of this function, we know that `ptr` came from
// a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
// holds a reference count increment that is transferrable to us.

> +        unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
> +    }
> +}
> +
>  impl<T: ?Sized> Deref for Arc<T> {
>      type Target = T;


Cheers,
Andreas