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Message-ID: <aBJTVWsrqzuasx7W@Mac.home>
Date: Wed, 30 Apr 2025 09:44:05 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Christian Schrefl <chrisi.schrefl@...il.com>
Cc: Sky <sky@...9.dev>, 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>,
Danilo Krummrich <dakr@...nel.org>,
Gerald Wisböck <gerald.wisboeck@...ther.ink>,
linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
Ralf Jung <post@...fj.de>
Subject: Re: [PATCH v2 3/3] rust: use `UnsafePinned` in the implementation of
`Opaque`
[Cc Ralf]
On Wed, Apr 30, 2025 at 10:36:13AM +0200, Christian Schrefl wrote:
> This change makes the semantics of the `Opaque` implementation
> clearer and prepares for the switch to the upstream rust UnsafePinned`
> type in the future.
>
> `Opaque` still uses `UnsafeCell` even though the kernel implementation
> of `UnsafePinned` already includes it, since the current upstream
> version does not.
>
> Reviewed-by: Gerald Wisböck <gerald.wisboeck@...ther.ink>
> Signed-off-by: Christian Schrefl <chrisi.schrefl@...il.com>
> ---
> rust/kernel/types.rs | 29 ++++++++++++-----------------
> 1 file changed, 12 insertions(+), 17 deletions(-)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index f06e8720e012102e5c41e79fd97b0607e927d71c..44d96423a8a6c358bb7ebf12c24fad98e5c2cb61 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -4,12 +4,12 @@
>
> use core::{
> cell::UnsafeCell,
> - marker::{PhantomData, PhantomPinned},
> + marker::PhantomData,
> mem::{ManuallyDrop, MaybeUninit},
> ops::{Deref, DerefMut},
> ptr::NonNull,
> };
> -use pin_init::{PinInit, Wrapper, Zeroable};
> +use pin_init::{cast_pin_init, PinInit, Wrapper, Zeroable};
>
> /// Used to transfer ownership to and from foreign (non-Rust) languages.
> ///
> @@ -308,8 +308,7 @@ fn drop(&mut self) {
> /// ```
> #[repr(transparent)]
> pub struct Opaque<T> {
> - value: UnsafeCell<MaybeUninit<T>>,
> - _pin: PhantomPinned,
> + value: UnsafePinned<UnsafeCell<MaybeUninit<T>>>,
What's the Rust upstream opinion on `&UnsafePinned` vs `&UnsafeCell`?
Does `&UnsafePinned` provide the same noalias behavior as `&UnsafeCell`?
I'm wondering whether we should just do:
pub struct Opaque<T> {
value: UnsafePinned<MaybeUninit<T>>,
_not_sync: PhantomData<UnsafeCell<()>>,
}
, instead.
Regards,
Boqun
> }
>
> // SAFETY: `Opaque<T>` allows the inner value to be any bit pattern, including all zeros.
> @@ -319,16 +318,14 @@ impl<T> Opaque<T> {
> /// Creates a new opaque value.
> pub const fn new(value: T) -> Self {
> Self {
> - value: UnsafeCell::new(MaybeUninit::new(value)),
> - _pin: PhantomPinned,
> + value: UnsafePinned::new(UnsafeCell::new(MaybeUninit::new(value))),
> }
> }
>
> /// Creates an uninitialised value.
> pub const fn uninit() -> Self {
> Self {
> - value: UnsafeCell::new(MaybeUninit::uninit()),
> - _pin: PhantomPinned,
> + value: UnsafePinned::new(UnsafeCell::new(MaybeUninit::uninit())),
> }
> }
>
> @@ -371,7 +368,7 @@ pub fn try_ffi_init<E>(
>
> /// Returns a raw pointer to the opaque data.
> pub const fn get(&self) -> *mut T {
> - UnsafeCell::get(&self.value).cast::<T>()
> + UnsafeCell::raw_get(self.value.get()).cast::<T>()
> }
>
> /// Gets the value behind `this`.
> @@ -384,14 +381,12 @@ pub const fn raw_get(this: *const Self) -> *mut T {
> }
> impl<T> Wrapper<T> for Opaque<T> {
> /// Create an opaque pin-initializer from the given pin-initializer.
> - fn pin_init<E>(slot: impl PinInit<T, E>) -> impl PinInit<Self, E> {
> - Self::try_ffi_init(|ptr: *mut T| {
> - // SAFETY:
> - // - `ptr` is a valid pointer to uninitialized memory,
> - // - `slot` is not accessed on error; the call is infallible,
> - // - `slot` is pinned in memory.
> - unsafe { PinInit::<T, E>::__pinned_init(slot, ptr) }
> - })
> + fn pin_init<E>(value_init: impl PinInit<T, E>) -> impl PinInit<Self, E> {
> + let value_init =
> + UnsafePinned::pin_init(UnsafeCell::pin_init(MaybeUninit::pin_init(value_init)));
> + // SAFETY: `Opaque<T>` is a `repr(transparent)` wrapper around
> + // `UnsafePinned<UnsafeCell<MabeUninit<T>>>` so the memory representation is compatible.
> + unsafe { cast_pin_init(value_init) }
> }
> }
>
>
> --
> 2.49.0
>
>
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