linux-kernel - [PATCH v3 2/4] rust: sync: reword the `Arc` safety comment for `Sync`

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Message-ID: <20230531145939.3714886-3-aliceryhl@google.com>
Date:   Wed, 31 May 2023 14:59:37 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     rust-for-linux@...r.kernel.org
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>,
        Benno Lossin <benno.lossin@...ton.me>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Martin Rodriguez Reboredo <yakoyoku@...il.com>
Subject: [PATCH v3 2/4] rust: sync: reword the `Arc` safety comment for `Sync`

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@...sung.com>
Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
Reviewed-by: Andreas Hindborg <a.hindborg@...sung.com>
Reviewed-by: Boqun Feng <boqun.feng@...il.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>
Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
---
 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 3ad831603cf4..3da84187ecf4 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 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 when
+// the reference count reaches zero and `T` is dropped.
 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 
 impl<T> Arc<T> {
-- 
2.41.0.rc0.172.g3f132b7071-goog