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SubjectRe: [PATCH 4/7] rust: sync: introduce `ArcBorrow`
Ops! I fall asleep while waiting for the Copy to derive Copy debate!

Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>


On Wed, Dec 28, 2022 at 7:05 AM Wedson Almeida Filho <wedsonaf@gmail.com> wrote:
>
> This allows us to create references to a ref-counted allocation without
> double-indirection and that still allow us to increment the refcount to
> a new `Arc<T>`.
>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> ---
> rust/kernel/sync.rs | 2 +-
> rust/kernel/sync/arc.rs | 97 +++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 98 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 39b379dd548f..5de03ea83ea1 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -7,4 +7,4 @@
>
> mod arc;
>
> -pub use arc::Arc;
> +pub use arc::{Arc, ArcBorrow};
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index dbc7596cc3ce..f68bfc02c81a 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -19,6 +19,7 @@ use crate::{bindings, error::Result, types::Opaque};
> use alloc::boxed::Box;
> use core::{
> marker::{PhantomData, Unsize},
> + mem::ManuallyDrop,
> ops::Deref,
> ptr::NonNull,
> };
> @@ -164,6 +165,18 @@ impl<T: ?Sized> Arc<T> {
> _p: PhantomData,
> }
> }
> +
> + /// Returns an [`ArcBorrow`] from the given [`Arc`].
> + ///
> + /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
> + /// receiver), but we have an [`Arc`] instead. Getting an [`ArcBorrow`] is free when optimised.
> + #[inline]
> + pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> {
> + // SAFETY: The constraint that the lifetime of the shared reference must outlive that of
> + // the returned `ArcBorrow` ensures that the object remains alive and that no mutable
> + // reference can be created.
> + unsafe { ArcBorrow::new(self.ptr) }
> + }
> }
>
> impl<T: ?Sized> Deref for Arc<T> {
> @@ -208,3 +221,87 @@ impl<T: ?Sized> Drop for Arc<T> {
> }
> }
> }
> +
> +/// A borrowed reference to an [`Arc`] instance.
> +///
> +/// For cases when one doesn't ever need to increment the refcount on the allocation, it is simpler
> +/// to use just `&T`, which we can trivially get from an `Arc<T>` instance.
> +///
> +/// However, when one may need to increment the refcount, it is preferable to use an `ArcBorrow<T>`
> +/// over `&Arc<T>` because the latter results in a double-indirection: a pointer (shared reference)
> +/// to a pointer (`Arc<T>`) to the object (`T`). An [`ArcBorrow`] eliminates this double
> +/// indirection while still allowing one to increment the refcount and getting an `Arc<T>` when/if
> +/// needed.
> +///
> +/// # Invariants
> +///
> +/// There are no mutable references to the underlying [`Arc`], and it remains valid for the
> +/// lifetime of the [`ArcBorrow`] instance.
> +///
> +/// # Example
> +///
> +/// ```
> +/// use crate::sync::{Arc, ArcBorrow};
> +///
> +/// struct Example;
> +///
> +/// fn do_something(e: ArcBorrow<'_, Example>) -> Arc<Example> {
> +/// e.into()
> +/// }
> +///
> +/// let obj = Arc::try_new(Example)?;
> +/// let cloned = do_something(obj.as_arc_borrow());
> +///
> +/// // Assert that both `obj` and `cloned` point to the same underlying object.
> +/// assert!(core::ptr::eq(&*obj, &*cloned));
> +/// ```
> +pub struct ArcBorrow<'a, T: ?Sized + 'a> {
> + inner: NonNull<ArcInner<T>>,
> + _p: PhantomData<&'a ()>,
> +}
> +
> +impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
> + fn clone(&self) -> Self {
> + *self
> + }
> +}
> +
> +impl<T: ?Sized> Copy for ArcBorrow<'_, T> {}
> +
> +impl<T: ?Sized> ArcBorrow<'_, T> {
> + /// Creates a new [`ArcBorrow`] instance.
> + ///
> + /// # Safety
> + ///
> + /// Callers must ensure the following for the lifetime of the returned [`ArcBorrow`] instance:
> + /// 1. That `inner` remains valid;
> + /// 2. That no mutable references to `inner` are created.
> + unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
> + // INVARIANT: The safety requirements guarantee the invariants.
> + Self {
> + inner,
> + _p: PhantomData,
> + }
> + }
> +}
> +
> +impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
> + fn from(b: ArcBorrow<'_, T>) -> Self {
> + // SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
> + // guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
> + // increment.
> + ManuallyDrop::new(unsafe { Arc::from_inner(b.inner) })
> + .deref()
> + .clone()
> + }
> +}
> +
> +impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
> + type Target = T;
> +
> + fn deref(&self) -> &Self::Target {
> + // SAFETY: By the type invariant, the underlying object is still alive with no mutable
> + // references to it, so it is safe to create a shared reference.
> + unsafe { &self.inner.as_ref().data }
> + }
> +}
> --
> 2.34.1
>

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