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From:   Andreas Hindborg <nmi@metaspace.dk>
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>, 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
Subject: Re: [PATCH v1 1/2] rust: specify when `ARef` is thread safe
Date:   Tue, 23 May 2023 15:11:13 +0200
In-reply-to: <20230517095905.3548100-1-aliceryhl@google.com>
Message-ID: <87y1lfdwei.fsf@metaspace.dk>
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Alice Ryhl <aliceryhl@google.com> writes:

> An `ARef` behaves just like the `Arc` when it comes to thread safety, so
> we can reuse the thread safety comments from `Arc` here.
>
> This is necessary because without this change, the Rust compiler will
> assume that things are not thread safe even though they are.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/types.rs | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 29db59d6119a..9c8d94c04deb 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -321,6 +321,17 @@ pub struct ARef<T: AlwaysRefCounted> {
>      _p: PhantomData<T>,
>  }
>  
> +// SAFETY: It is safe to send `ARef<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 an `ARef<T>` may ultimately access `T` directly, for
> +// example, when the reference count reaches zero and `T` is dropped.
> +unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
> +
> +// SAFETY: It is safe to send `&ARef<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 `&ARef<T>`
> +// into an `ARef<T>`, which may lead to `T` being accessed by the same reasoning as above.
> +unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}

Nit: I would prefer repeating the safety comment details, in case the
two drift apart in the future.


BR Andreas

> +
>  impl<T: AlwaysRefCounted> ARef<T> {
>      /// Creates a new instance of [`ARef`].
>      ///
>
> base-commit: ac9a78681b921877518763ba0e89202254349d1b