linux-kernel - Re: [PATCH v4 1/3] rust: add UnsafePinned type

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Message-Id: <D9VBVURZLSNT.4BTQQ8UCTGPJ@kernel.org>
Date: Tue, 13 May 2025 22:51:03 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Christian Schrefl" <chrisi.schrefl@...il.com>, "Sky" <sky@...9.dev>,
 "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>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>, Gerald Wisböck
 <gerald.wisboeck@...ther.ink>, "Nathan Chancellor" <nathan@...nel.org>,
 "Nick Desaulniers" <nick.desaulniers+lkml@...il.com>, "Bill Wendling"
 <morbo@...gle.com>, "Justin Stitt" <justinstitt@...gle.com>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <llvm@...ts.linux.dev>
Subject: Re: [PATCH v4 1/3] rust: add UnsafePinned type

On Sun May 11, 2025 at 8:21 PM CEST, Christian Schrefl wrote:
> `UnsafePinned<T>` is useful for cases where a value might be shared with
> C code but not directly used by it. In particular this is added for
> storing additional data in the `MiscDeviceRegistration` which will be
> shared between `fops->open` and the containing struct.
>
> Similar to `Opaque` but guarantees that the value is always initialized
> and that the inner value is dropped when `UnsafePinned` is dropped.
>
> This was originally proposed for the IRQ abstractions [0] and is also
> useful for other where the inner data may be aliased, but is always
> valid and automatic `Drop` is desired.
>
> Since then the `UnsafePinned` type was added to upstream Rust [1] by Sky
> as a unstable feature, therefore this patch implements the subset of the
> upstream API for the `UnsafePinned` type required for additional data in
> `MiscDeviceRegistration` and in the implementation of the `Opaque` type.
>
> Some differences to the upstream type definition are required in the
> kernel implementation, because upstream type uses some compiler changes
> to opt out of certain optimizations, this is documented in the
> documentation and a comment on the `UnsafePinned` type.
>
> The documentation on is based on the upstream rust documentation with
> minor modifications for the kernel implementation.
>
> Link: https://lore.kernel.org/rust-for-linux/CAH5fLgiOASgjoYKFz6kWwzLaH07DqP2ph+3YyCDh2+gYqGpABA@mail.gmail.com [0]
> Link: https://github.com/rust-lang/rust/pull/137043 [1]
> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
> Reviewed-by: Gerald Wisböck <gerald.wisboeck@...ther.ink>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Co-developed-by: Sky <sky@...9.dev>
> Signed-off-by: Sky <sky@...9.dev>
> Signed-off-by: Christian Schrefl <chrisi.schrefl@...il.com>

One nit below, with that fixed:

Reviewed-by: Benno Lossin <lossin@...nel.org>

> ---
>  rust/kernel/types.rs               |   6 ++
>  rust/kernel/types/unsafe_pinned.rs | 111 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 117 insertions(+)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 9d0471afc9648f2973235488b441eb109069adb1..705f420fdfbc4a576de1c4546578f2f04cdf615e 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -578,3 +581,6 @@ pub enum Either<L, R> {
>  /// [`NotThreadSafe`]: type@...ThreadSafe
>  #[allow(non_upper_case_globals)]
>  pub const NotThreadSafe: NotThreadSafe = PhantomData;
> +
> +mod unsafe_pinned;
> +pub use unsafe_pinned::UnsafePinned;

I would put `mod` to the top of the 

---
Cheers,
Benno