Re: [PATCH v2 2/4] rust: sync: reword the `Arc` safety comment for `Sync`

[Gary Guo]

On Tue, 23 May 2023 14:44:16 +0000
Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:

> The safety comment on `impl Sync for Arc` references the Send safety
> comment. This commit avoids that in case the two comments drift apart in
> the future.
> 
> Suggested-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
> Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---
>  rust/kernel/sync/arc.rs | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index 87a4c9ed712b..4d10f9868d9e 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -150,9 +150,11 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Arc<U>> for Ar
>  // mutable reference, for example, when the reference count reaches zero and `T` is dropped.
>  unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
>  
> -// SAFETY: It is safe to send `&Arc<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 `&Arc<T>`
> -// into an `Arc<T>`, which may lead to `T` being accessed by the same reasoning as above.
> +// SAFETY: It is safe to send `&Arc<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 a `&Arc<T>` may clone it and get an
> +// `Arc<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
> +// example, when the reference count reaches zero and `T` is dropped.

"for example" here implies that there are other case to get a mutable
reference? I don't think that's true for our `Arc` since we don't
provide a `get_mut` method.

>  unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
>  
>  impl<T> Arc<T> {