linux-kernel - Re: [PATCH v2 4/4] rust: task: add `Send` marker to `Task`

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Date:   Thu, 25 May 2023 13:46:40 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     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 4/4] rust: task: add `Send` marker to `Task`

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

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

> ---
>   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 {}
> 
> --
> 2.40.1.698.g37aff9b760-goog
> 

-- 
Cheers,
Benno