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Message-ID: <c7c3c740-b534-6be1-aead-dc8dbba80882@gmail.com>
Date:   Tue, 23 May 2023 13:29:07 -0300
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Subject: Re: [PATCH v2 2/4] rust: sync: reword the `Arc` safety comment for
 `Sync`
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To:     Alice Ryhl <aliceryhl@google.com>, Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>
Cc:     Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=c3=b6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Andreas Hindborg <a.hindborg@samsung.com>
References: <20230523144418.1250547-1-aliceryhl@google.com>
 <20230523144418.1250547-3-aliceryhl@google.com>
From:   Martin Rodriguez Reboredo <yakoyoku@gmail.com>
In-Reply-To: <20230523144418.1250547-3-aliceryhl@google.com>
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On 5/23/23 11:44, Alice Ryhl wrote:
> The safety comment on `impl Sync for Arc` references the Send safety
> comment. This commit avoids that in case the two comments drift apart in
> the future.
> 
> Suggested-by: Andreas Hindborg <a.hindborg@samsung.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> [...]
>   
> -// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `T` is `Sync` for the
> -// same reason as above. `T` needs to be `Send` as well because a thread can clone an `&Arc<T>`
> -// into an `Arc<T>`, which may lead to `T` being accessed by the same reasoning as above.
> +// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `T` is `Sync`
> +// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
> +// it needs `T` to be `Send` because any thread that has a `&Arc<T>` may clone it and get an
> +// `Arc<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
> +// example, when the reference count reaches zero and `T` is dropped.
>   unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
>   
>   impl<T> Arc<T> {

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>