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    SubjectRe: [PATCH v2 2/4] rust: sync: reword the `Arc` safety comment for `Sync`
    On Tue, 23 May 2023 14:44:16 +0000
    Alice Ryhl <aliceryhl@google.com> 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@samsung.com>
    > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
    > Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
    > Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
    > ---
    > 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> {

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