linux-kernel - Re: [PATCH] rust: arc: remove unused PhantomData

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Message-ID: <ZylNvC8enwPG4IQ4@Boquns-Mac-mini.local>
Date: Mon, 4 Nov 2024 14:42:04 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: arc: remove unused PhantomData

Hi Tamir,

On Mon, Nov 04, 2024 at 05:23:44PM -0400, Tamir Duberstein wrote:
> There's no need for this. The type had the same form when it was first

I believe we need this `PhantomData` to inform drop chec [1] that the
drop of `Arc` may result into the drop of an `ArcInner<T>`. Rust std
`Arc` has the similar definition [2], you can find more information
about PhantomData usage on drop checking at [3].

[1]: https://doc.rust-lang.org/nightly/std/ops/trait.Drop.html#drop-check
[2]: https://doc.rust-lang.org/src/alloc/sync.rs.html#245
[3]: https://doc.rust-lang.org/nightly/std/marker/struct.PhantomData.html#ownership-and-the-drop-check

Regards,
Boqun

> introduced, so it seems this was never necessary.
> 
> Fixed: 9dc043655003 ("rust: sync: add `Arc` for ref-counted allocations")
> Signed-off-by: Tamir Duberstein <tamird@...il.com>
> ---
>  rust/kernel/sync/arc.rs | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index db9da352d588f65348aa7a5204abbb165b70197f..7e54d31538273d410f80fd65b2070e75e4f69428 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -127,7 +127,6 @@
>  /// ```
>  pub struct Arc<T: ?Sized> {
>      ptr: NonNull<ArcInner<T>>,
> -    _p: PhantomData<ArcInner<T>>,
>  }
>  
>  #[pin_data]
> @@ -219,7 +218,6 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
>          // INVARIANT: By the safety requirements, the invariants hold.
>          Arc {
>              ptr: inner,
> -            _p: PhantomData,
>          }
>      }
>  
> 
> ---
> base-commit: ae7851c29747fa3765ecb722fe722117a346f988
> change-id: 20241104-simplify-arc-70c3574b5fac
> 
> Best regards,
> -- 
> Tamir Duberstein <tamird@...il.com>
>