Re: [PATCH 4/7] rust: sync: introduce `ArcBorrow`

From: Emilio Cobos Álvarez
Date: Wed Jan 04 2023 - 11:07:43 EST

Sorry for the drive-by comment, but maybe it saves some work.

On 1/4/23 16:29, Wedson Almeida Filho wrote:
On Sat, 31 Dec 2022 at 19:43, Gary Guo <gary@xxxxxxxxxxx> wrote:

On Wed, 28 Dec 2022 06:03:43 +0000
Wedson Almeida Filho <wedsonaf@xxxxxxxxx> 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@xxxxxxxxx>
---
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> {}

Couldn't this just be derived `Clone` and `Copy`?

Indeed. I'll send a v2 with this.

I'm not sure this is true. Deriving will add the T: Copy and T: Clone bound, which I think is not what you want here.

i.e., I assume you want an ArcBorrow to be Copy even if the underlying T is not.

See <https://github.com/rust-lang/rust/issues/26925> for the relevant (really long-standing) Rust issue.

Cheers,

-- Emilio