Re: [PATCH v2 5/5] rust: types: implement `ForeignOwnable` for `Arc`

Andreas Hindborg
2023-02-01T10:36:00Z

Wedson Almeida Filho <wedsonaf@gmail.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@gmail.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
> Reviewed-by: Alice Ferrazzi <alice.ferrazzi@miraclelinux.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