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Date:   Thu, 25 May 2023 13:45:12 +0000
To:     Martin Rodriguez Reboredo <yakoyoku@gmail.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Alice Ryhl <aliceryhl@google.com>, 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>,
        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>
Subject: Re: [PATCH v2 3/4] rust: specify when `ARef` is thread safe
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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.
>=20
> This is necessary because without this change, the Rust compiler will
> assume that things are not thread safe even though they are.
>=20
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

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

> ---
>   rust/kernel/types.rs | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>=20
> 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>,
>   }
>=20
> +// SAFETY: It is safe to send `ARef<T>` to another thread when the under=
lying `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 ultimat=
ely 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 unde=
rlying `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>` ma=
y clone it and get an
> +// `ARef<T>` on that thread, so the thread may ultimately access `T` usi=
ng 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
>=20

--=20
Cheers,
Benno