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Date:   Tue, 23 May 2023 19:08:50 +0200
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Subject: Re: [PATCH v2 2/4] rust: sync: reword the `Arc` safety comment for
 `Sync`
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To:     Gary Guo <gary@garyguo.net>
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?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>,
        Alice Ryhl <aliceryhl@google.com>
References: <20230523144418.1250547-1-aliceryhl@google.com>
 <20230523144418.1250547-3-aliceryhl@google.com>
 <20230523165027.71600b4b.gary@garyguo.net>
From:   Alice Ryhl <alice@ryhl.io>
In-Reply-To: <20230523165027.71600b4b.gary@garyguo.net>
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On 5/23/23 17:50, Gary Guo wrote:
>> -// 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.
> 
> "for example" here implies that there are other case to get a mutable
> reference? I don't think that's true for our `Arc` since we don't
> provide a `get_mut` method.

Ah, yes, that's true. Good point.