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Date:   Tue, 23 May 2023 17:00:55 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Alice Ryhl <aliceryhl@google.com>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "=?UTF-8?B?QmrDtnJu?= Roy Baron" <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev,
        Wedson Almeida Filho <walmeida@microsoft.com>
Subject: Re: [PATCH v1 1/2] rust: sync: add `Arc::ptr_eq`
Message-ID: <20230523170055.4e2ef9a9.gary@garyguo.net>
In-Reply-To: <20230517200814.3157916-1-aliceryhl@google.com>
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On Wed, 17 May 2023 20:08:13 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> Add a method for comparing whether two `Arc` pointers reference the same
> underlying object.
> 
> This comparison can already be done by getting a reference to the inner
> values and comparing whether the references have the same address.
> However, writing `Arc::ptr_eq(a, b)` is generally less error-prone than
> doing the same check on the references, since you might otherwise
> accidentally compare the two `&Arc<T>` references instead, which wont
> work because those are pointers to pointers to the inner value, when you
> just want to compare the pointers to the inner value.
> 
> Also, this method might optimize better because getting a reference to
> the inner value involves offsetting the pointer, which this method does
> not need to do.
> 
> Co-developed-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/sync/arc.rs | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index e6d206242465..274febe3bb06 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -221,6 +221,11 @@ impl<T: ?Sized> Arc<T> {
>          // reference can be created.
>          unsafe { ArcBorrow::new(self.ptr) }
>      }
> +
> +    /// Compare whether two [`Arc`] pointers reference the same underlying object.
> +    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
> +        core::ptr::eq(this.ptr.as_ptr(), other.ptr.as_ptr())
> +    }
>  }
>  
>  impl<T: 'static> ForeignOwnable for Arc<T> {
> 
> base-commit: ac9a78681b921877518763ba0e89202254349d1b