diff mbox series

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

Message ID 20230523144418.1250547-3-aliceryhl@google.com
State New
Headers
Series Update thread safety markers |

Commit Message

Alice Ryhl May 23, 2023, 2:44 p.m. UTC 
  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(-)

Comments

Gary Guo May 23, 2023, 3:50 p.m. UTC | #1 
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> {
Martin Rodriguez Reboredo May 23, 2023, 4:29 p.m. UTC | #2 
On 5/23/23 11:44, Alice Ryhl 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>
> ---
> [...]
>   
> -// 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.
>   unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
>   
>   impl<T> Arc<T> {

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Alice Ryhl May 23, 2023, 5:08 p.m. UTC | #3 
On 5/23/23 17:50, Gary Guo wrote:
>> -// 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.

Ah, yes, that's true. Good point.
Benno Lossin May 25, 2023, 1:43 p.m. UTC | #4 
On 5/23/23 16:44, Alice Ryhl 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>

Reviewed-by: Benno Lossin <benno.lossin@proton.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 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.
>   unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
> 
>   impl<T> Arc<T> {
> --
> 2.40.1.698.g37aff9b760-goog
>
diff mbox series

Patch

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