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Date:   Thu, 02 Feb 2023 16:53:19 +0000
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To:     Boqun Feng <boqun.feng@gmail.com>
From:   =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Gary Guo <gary@garyguo.net>,
        Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Subject: Re: [RFC 3/5] rust: sync: Arc: Introduces Arc::get_inner() helper
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In-Reply-To: <20230201232244.212908-4-boqun.feng@gmail.com>
References: <20230201232244.212908-1-boqun.feng@gmail.com> <20230201232244.212908-4-boqun.feng@gmail.com>
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On Thursday, February 2nd, 2023 at 00:22, Boqun Feng <boqun.feng@gmail.com>=
 wrote:


> Getting a `&ArcInner<T>` should always be safe from a `Arc<T>`,
>=20
> therefore add this helper function to avoid duplicate unsafe
> `ptr.as_ref()`.
>=20
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Reviewed-by: Bj=C3=B6rn Roy Baron <bjorn3_gh@protonmail.com>

> ---
>  rust/kernel/sync/arc.rs | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
>=20
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index fbfceaa3096e..4d8de20c996f 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -201,6 +201,13 @@ impl<T: ?Sized> Arc<T> {
>          // reference can be created.
>          unsafe { ArcBorrow::new(self.ptr) }
>      }
> +
> +    /// Returns a reference to the internal [`ArcInner`].
> +    fn get_inner(&self) -> &ArcInner<T> {
> +        // SAFETY: By the type invariant, there is necessarily a referen=
ce to the object, so it is
> +        // safe to dereference it.
> +        unsafe { self.ptr.as_ref() }
> +    }
>  }
>=20
>  impl<T: 'static> ForeignOwnable for Arc<T> {
> @@ -233,9 +240,7 @@ impl<T: ?Sized> Deref for Arc<T> {
>      type Target =3D T;
>=20
>      fn deref(&self) -> &Self::Target {
> -        // SAFETY: By the type invariant, there is necessarily a referen=
ce to the object, so it is
> -        // safe to dereference it.
> -        unsafe { &self.ptr.as_ref().data }
> +        &self.get_inner().data
>      }
>  }
>=20
> @@ -244,7 +249,7 @@ impl<T: ?Sized> Clone for Arc<T> {
>          // INVARIANT: C `refcount_inc` saturates the refcount, so it can=
not overflow to zero.
>          // SAFETY: By the type invariant, there is necessarily a referen=
ce to the object, so it is
>          // safe to increment the refcount.
> -        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()=
) };
> +        unsafe { bindings::refcount_inc(self.get_inner().refcount.get())=
 };
>=20
>          // SAFETY: We just incremented the refcount. This increment is n=
ow owned by the new `Arc`.
>          unsafe { Self::from_inner(self.ptr) }
> @@ -253,11 +258,7 @@ impl<T: ?Sized> Clone for Arc<T> {
>=20
>  impl<T: ?Sized> Drop for Arc<T> {
>      fn drop(&mut self) {
> -        // SAFETY: By the type invariant, there is necessarily a referen=
ce to the object. We cannot
> -        // touch `refcount` after it's decremented to a non-zero value b=
ecause another thread/CPU
> -        // may concurrently decrement it to zero and free it. It is ok t=
o have a raw pointer to
> -        // freed/invalid memory as long as it is never dereferenced.
> -        let refcount =3D unsafe { self.ptr.as_ref() }.refcount.get();
> +        let refcount =3D self.get_inner().refcount.get();
>=20
>          // INVARIANT: If the refcount reaches zero, there are no other i=
nstances of `Arc`, and
>          // this instance is being dropped, so the broken invariant is no=
t observable.
> --
> 2.39.1