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Date:   Tue, 23 May 2023 16:56:29 +0100
From:   Gary Guo <gary@garyguo.net>
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>,
        "=?UTF-8?B?QmrDtnJu?= 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, Andreas Hindborg <a.hindborg@samsung.com>
Subject: Re: [PATCH v2 4/4] rust: task: add `Send` marker to `Task`
Message-ID: <20230523165629.0b092b08.gary@garyguo.net>
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On Tue, 23 May 2023 14:44:18 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> 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>
> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/task.rs | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 526d29a0ae27..7eda15e5f1b3 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -64,8 +64,14 @@ macro_rules! current {
>  #[repr(transparent)]
>  pub struct Task(pub(crate) Opaque<bindings::task_struct>);
>  
> -// 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
> +// SAFETY: By design, the only way to access a `Task` is via the `current` function or via an
> +// `ARef<Task>` obtained through the `AlwaysRefCounted` impl. This means that the only situation in
> +// which a `Task` 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 {}
> +
> +// SAFETY: It's OK to access `Task` through shared 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`).
>  unsafe impl Sync for Task {}
>