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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 2/2] rust: task: add `Send` marker to `Task`
Date:   Tue, 23 May 2023 15:27:14 +0200
In-reply-to: <20230517095905.3548100-2-aliceryhl@google.com>
Message-ID: <87r0r7dunu.fsf@metaspace.dk>
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Alice Ryhl <aliceryhl@google.com> writes:

> When a type also implements `Sync`, the meaning of `Send` is just "this
> type may be accessed mutably from threads other than the one it is
> created on". That's ok for this type.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/task.rs | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 526d29a0ae27..4f1fe9aa9f6e 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -64,6 +64,11 @@ macro_rules! current {
>  #[repr(transparent)]
>  pub struct Task(pub(crate) Opaque<bindings::task_struct>);
>  
> +// SAFETY: The only situation in which this can be accessed mutably is when the refcount drops to
> +// zero and the destructor runs. It is safe for that to happen on any thread, so it is ok for this
> +// type to be `Send`.
> +unsafe impl Send for Task {}

To enhance clarity, could you elaborate _why_ `Task` can never be
accessed mutably by Rust? Perhaps "By design, `Task` can only be
accessed thorough `&Task` and `Task` can never be owned by the Rust
side. Therefore the only situation ...".

> +
>  // SAFETY: It's OK to access `Task` through references from other threads because we're either
>  // accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
>  // synchronised by C code (e.g., `signal_pending`).