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Message-ID: <9683fcd6-4fdc-41f7-bfc6-78a44d7360e8@gmail.com>
Date: Wed, 30 Apr 2025 19:07:53 +0200
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Boqun Feng <boqun.feng@...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`
On 30.04.25 6:44 PM, Boqun Feng wrote:
> [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`?
>From the upsteam rust docs [0]:
> Further note that this does not lift the requirement that shared
references must be read-only! Use `UnsafeCell` for that.
I at some point there was discussion about possibly needing to use
`UnsafeCell` internally (because of `pin::deref`). But since the
current upstream documentation explicitly mentions still needing
`UnsafeCell` I assume that it will stay this was. If the upstream
implementation changes I can add a patch that removes the
unnecessary `UnsafeCell` and changes the documentation.
I asked about this on the `UnsafePinned` rust tracking issue [1].
Link: https://doc.rust-lang.org/nightly/std/pin/struct.UnsafePinned.html [0]
Link: https://github.com/rust-lang/rust/issues/125735#issuecomment-2842668816 [1]
Cheers
Christian
> 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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