linux-kernel - Re: [PATCH v1 1/2] rust: specify when `ARef` is thread safe

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Date:   Thu, 18 May 2023 14:24:05 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Gary Guo <gary@...yguo.net>,
        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
Subject: Re: [PATCH v1 1/2] rust: specify when `ARef` is thread safe

On Wed, May 17, 2023 at 09:59:04AM +0000, 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>
> ---
>  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

Does the "ultimately access `T` directly" here imply mutably or
exclusively? If so, it makes sense to me to call it out. I'm trying to
make sure we can agree on some "common terminologies" ;-)

Regards,
Boqun

> +// 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> {}
> +
>  impl<T: AlwaysRefCounted> ARef<T> {
>      /// Creates a new instance of [`ARef`].
>      ///
> 
> base-commit: ac9a78681b921877518763ba0e89202254349d1b
> -- 
> 2.40.1.606.ga4b1b128d6-goog
>