linux-kernel - Re: [PATCH v5 1/4] rust: sync: change `<Arc<T> as ForeignOwnable>::PointedTo` to `T`

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Message-ID: <87ikovp7l7.fsf@kernel.org>
Date: Thu, 27 Feb 2025 19:26:28 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Tamir Duberstein" <tamird@...il.com>
Cc: "Danilo Krummrich" <dakr@...nel.org>,  "Miguel Ojeda"
 <ojeda@...nel.org>,  "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>,  "Trevor Gross" <tmgross@...ch.edu>,
  "Joel Becker" <jlbec@...lplan.org>,  "Peter Zijlstra"
 <peterz@...radead.org>,  "Ingo Molnar" <mingo@...hat.com>,  "Will Deacon"
 <will@...nel.org>,  "Waiman Long" <longman@...hat.com>,  "Fiona Behrens"
 <me@...enk.dev>,  "Charalampos Mitrodimas" <charmitro@...teo.net>,
  "Daniel Almeida" <daniel.almeida@...labora.com>,
  <rust-for-linux@...r.kernel.org>,  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/4] rust: sync: change `<Arc<T> as
 ForeignOwnable>::PointedTo` to `T`

Andreas Hindborg <a.hindborg@...nel.org> writes:

> "Tamir Duberstein" <tamird@...il.com> writes:
>
>> On Thu, Feb 27, 2025 at 7:37 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>>
>>> Using `ArcInner` as `PoinedTo` in the `ForeignOwnable` implementation for
>>> `Arc` is a bit unfortunate. Using `T` as `PointedTo` does not remove any
>>> functionality, but allows `ArcInner` to be private. Further, it allows
>>> downstream users to write code that is generic over `Box` and `Arc`, when
>>> downstream users need access to `T` after calling `into_foreign`.
>>>
>>> Reviewed-by: Fiona Behrens <me@...enk.dev>
>>> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
>>> Tested-by: Daniel Almeida <daniel.almeida@...labora.com>
>>> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
>>> ---
>>>
>>> This patch is a dependency for Rust `configfs` abstractions. It allows both
>>> `Box` and `Arc` to be used as pointer types in the `configfs` hierarchy.
>>> ---
>>>  rust/kernel/sync/arc.rs | 21 ++++++++++++++++-----
>>>  1 file changed, 16 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
>>> index dfe4abf82c25..3d77a31e116f 100644
>>> --- a/rust/kernel/sync/arc.rs
>>> +++ b/rust/kernel/sync/arc.rs
>>> @@ -143,7 +143,7 @@ pub struct Arc<T: ?Sized> {
>>>  #[doc(hidden)]
>>>  #[pin_data]
>>>  #[repr(C)]
>>> -pub struct ArcInner<T: ?Sized> {
>>> +struct ArcInner<T: ?Sized> {
>>>      refcount: Opaque<bindings::refcount_t>,
>>>      data: T,
>>>  }
>>> @@ -345,18 +345,25 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
>>>
>>>  // SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
>>>  unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
>>> -    type PointedTo = ArcInner<T>;
>>> +    type PointedTo = T;
>>>      type Borrowed<'a> = ArcBorrow<'a, T>;
>>>      type BorrowedMut<'a> = Self::Borrowed<'a>;
>>>
>>>      fn into_foreign(self) -> *mut Self::PointedTo {
>>> -        ManuallyDrop::new(self).ptr.as_ptr()
>>> +        let this = ManuallyDrop::new(self).ptr.as_ptr();
>>> +        // SAFETY: `x` is a valid pointer to `Self` so the projection below is
>>> +        // in bounds of the allocation.
>>
>> Isn't the unsafe bit `*this`, which is what this comment should
>> justify?
>
> Yes, not sure what the origin of that name is. Good catch.

Referring to the `x` part here.


Best regards,
Andreas Hindborg