Date:   Wed, 4 Jan 2023 15:29:48 +0000
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Gary Guo <gary@...yguo.net>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] rust: sync: introduce `ArcBorrow`

On Sat, 31 Dec 2022 at 19:43, Gary Guo <gary@...yguo.net> wrote:
>
> On Wed, 28 Dec 2022 06:03:43 +0000
> Wedson Almeida Filho <wedsonaf@...il.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@...il.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> {}
>
> Couldn't this just be derived `Clone` and `Copy`?

Indeed. I'll send a v2 with this.

>
> > +
> > +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()
> > +    }
> > +}
>
> It might be easier to follow if this is jsut `bindings::refcount_inc`
> followed by `Arc::from_inner`?

I'd prefer to keep the interactions with `refcount_t` in `Arc` only so
that we can more easily change it in the future if we so choose.

> > +
> > +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 }
> > +    }
> > +}

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