linux-kernel - Re: [PATCH v2 2/4] rust: sync: reword the `Arc` safety comment for `Sync`

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Date:   Tue, 23 May 2023 19:08:50 +0200
From:   Alice Ryhl <alice@...l.io>
To:     Gary Guo <gary@...yguo.net>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Andreas Hindborg <a.hindborg@...sung.com>,
        Alice Ryhl <aliceryhl@...gle.com>
Subject: Re: [PATCH v2 2/4] rust: sync: reword the `Arc` safety comment for
 `Sync`

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.