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

[Wedson Almeida Filho]

On Wed, 4 Jan 2023 at 16:06, Emilio Cobos Álvarez <emilio@xxxxxxxxx> wrote:
>
> 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.

Thanks for the heads up, Emilio!

After trying this out, derive doesn't work. The errors brought me back
memories of when I first implemented this over a year ago, though I
didn't take the time to try to understand why it was failing.

So no v2. The series will remain as is.

Cheers

>
> Cheers,
>
>   -- Emilio