linux-kernel - Re: [PATCH v2 3/4] rust: specify when `ARef` is thread safe

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Date:   Thu, 25 May 2023 13:45:12 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Martin Rodriguez Reboredo <yakoyoku@...il.com>
Cc:     Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        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>
Subject: Re: [PATCH v2 3/4] rust: specify when `ARef` is thread safe

On 5/23/23 16:44, Alice Ryhl wrote:
> 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@...gle.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@...sung.com>
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>

> ---
>   rust/kernel/types.rs | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 29db59d6119a..1e5380b16ed5 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -321,6 +321,19 @@ 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` using a
> +// mutable reference, 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`
> +// 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 `&ARef<T>` may clone it and get an
> +// `ARef<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: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
> +
>   impl<T: AlwaysRefCounted> ARef<T> {
>       /// Creates a new instance of [`ARef`].
>       ///
> --
> 2.40.1.698.g37aff9b760-goog
> 

-- 
Cheers,
Benno