linux-kernel - Re: [PATCH v2 1/3] rust: add UnsafePinned type

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Message-ID: <aBHrO_hE0CEb0Wfw@google.com>
Date: Wed, 30 Apr 2025 09:19:55 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Christian Schrefl <chrisi.schrefl@...il.com>
Cc: 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>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, 
	"Gerald Wisböck" <gerald.wisboeck@...ther.ink>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 1/3] rust: add UnsafePinned type

On Wed, Apr 30, 2025 at 10:36:11AM +0200, 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>
> Co-developed-by: Sky <sky@...9.dev>
> Signed-off-by: Sky <sky@...9.dev>
> Signed-off-by: Christian Schrefl <chrisi.schrefl@...il.com>

> +config RUSTC_HAS_UNSAFE_PINNED
> +	def_bool RUSTC_VERSION >= 108800

> +#![cfg_attr(CONFIG_RUSTC_HAS_UNSAFE_PINNED, feature(unsafe_pinned))]

Actually, I missed this on the first look. Where is this feature used?
You only have a re-implementation right now.

Alice