From: Gary Guo <gary@garyguo.net>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org,
lkmm@lists.linux.dev, linux-arch@vger.kernel.org,
"Miguel Ojeda" <ojeda@kernel.org>,
"Alex Gaynor" <alex.gaynor@gmail.com>,
"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
"Benno Lossin" <lossin@kernel.org>,
"Andreas Hindborg" <a.hindborg@kernel.org>,
"Alice Ryhl" <aliceryhl@google.com>,
"Trevor Gross" <tmgross@umich.edu>,
"Danilo Krummrich" <dakr@kernel.org>,
"Will Deacon" <will@kernel.org>,
"Peter Zijlstra" <peterz@infradead.org>,
"Mark Rutland" <mark.rutland@arm.com>,
"Wedson Almeida Filho" <wedsonaf@gmail.com>,
"Viresh Kumar" <viresh.kumar@linaro.org>,
"Lyude Paul" <lyude@redhat.com>, "Ingo Molnar" <mingo@kernel.org>,
"Mitchell Levy" <levymitchell0@gmail.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Linus Torvalds" <torvalds@linux-foundation.org>,
"Thomas Gleixner" <tglx@linutronix.de>
Subject: Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics
Date: Mon, 23 Jun 2025 19:30:19 +0100 [thread overview]
Message-ID: <20250623193019.6c425467.gary@garyguo.net> (raw)
In-Reply-To: <aFjj8AV668pl9jLN@Mac.home>
On Sun, 22 Jun 2025 22:19:44 -0700
Boqun Feng <boqun.feng@gmail.com> wrote:
> On Sat, Jun 21, 2025 at 12:32:12PM +0100, Gary Guo wrote:
> [...]
> > > +#[repr(transparent)]
> > > +pub struct Atomic<T: AllowAtomic>(Opaque<T>);
> >
> > This should store `Opaque<T::Repr>` instead.
> >
>
> "should" is a strong word ;-) If we still use `into_repr`/`from_repr`
> it's a bit impossible, because Atomic::new() wants to be a const
> function, so it requires const_trait_impl I believe.
>
> If we require transmutability as a safety requirement for `AllowAtomic`,
> then either `T` or `T::Repr` is fine.
>
> > The implementation below essentially assumes that this is
> > `Opaque<T::Repr>`:
> > * atomic ops cast this to `*mut T::Repr`
> > * load/store operates on `T::Repr` then converts to `T` with
> > `T::from_repr`/`T::into_repr`.
> >
>
> Note that we only require one direction of strong transmutability, that
> is: for every `T`, it must be able to safe transmute to a `T::Repr`, for
> `T::Repr` -> `T` transmutation, only if it's a result of a `transmute<T,
> T::Repr>()`. This is mostly due to potential support for unit-only enum.
> E.g. using an atomic variable to represent a finite state.
>
> > Note tha the transparent new types restriction on `AllowAtomic` is not
> > sufficient for this, as I can define
> >
>
> Nice catch! I do agree we should disallow `MyWeirdI32`, and I also agree
> that we should put transmutability as safety requirement for
> `AllowAtomic`. However, I would suggest we still keep
> `into_repr`/`from_repr`, and require the implementation to make them
> provide the same results as transmute(), as a correctness precondition
> (instead of a safety precondition), in other words, you can still write
> a `MyWeirdI32`, and it won't cause safety issues, but it'll be
> incorrect.
>
> The reason why I think we should keep `into_repr`/`from_repr` but add
> a correctness precondition is that they are easily to implement as safe
> code for basic types, so it'll be better than a transmute() call. Also
> considering `Atomic<*mut T>`, would transmuting between integers and
> pointers act the same as expose_provenance() and
> from_exposed_provenance()?
Okay, this is more problematic than I thought then. For pointers, you
cannot just transmute between from pointers to usize (which is its
Repr):
* Transmuting from pointer to usize discards provenance
* Transmuting from usize to pointer gives invalid provenance
We want neither behaviour, so we must store `usize` directly and
always call into repr functions.
To make things cost I guess you would need an extra trait to indicate
that transmuting is fine.
Best,
Gary
next prev parent reply other threads:[~2025-06-23 18:30 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-06-18 16:49 [PATCH v5 00/10] LKMM generic atomics in Rust Boqun Feng
2025-06-18 16:49 ` [PATCH v5 01/10] rust: Introduce atomic API helpers Boqun Feng
2025-06-26 8:44 ` Andreas Hindborg
2025-06-27 14:00 ` Boqun Feng
2025-06-18 16:49 ` [PATCH v5 02/10] rust: sync: Add basic atomic operation mapping framework Boqun Feng
2025-06-26 8:50 ` Andreas Hindborg
2025-06-26 10:17 ` Andreas Hindborg
2025-06-27 14:30 ` Boqun Feng
2025-06-18 16:49 ` [PATCH v5 03/10] rust: sync: atomic: Add ordering annotation types Boqun Feng
2025-06-19 10:31 ` Peter Zijlstra
2025-06-19 12:19 ` Alice Ryhl
2025-06-19 13:29 ` Boqun Feng
2025-06-19 14:32 ` Peter Zijlstra
2025-06-19 15:00 ` Boqun Feng
2025-06-19 15:10 ` Peter Zijlstra
2025-06-19 15:15 ` Boqun Feng
2025-06-19 18:04 ` Alan Stern
2025-06-21 11:18 ` Gary Guo
2025-06-23 2:48 ` Boqun Feng
2025-06-26 12:36 ` Andreas Hindborg
2025-06-27 14:34 ` Boqun Feng
2025-06-27 14:44 ` Boqun Feng
2025-06-18 16:49 ` [PATCH v5 04/10] rust: sync: atomic: Add generic atomics Boqun Feng
2025-06-21 11:32 ` Gary Guo
2025-06-23 5:19 ` Boqun Feng
2025-06-23 11:54 ` Benno Lossin
2025-06-23 12:58 ` Boqun Feng
2025-06-23 18:30 ` Gary Guo [this message]
2025-06-23 19:09 ` Boqun Feng
2025-06-23 23:27 ` Benno Lossin
2025-06-24 16:35 ` Boqun Feng
2025-06-26 13:54 ` Benno Lossin
2025-07-04 21:22 ` Boqun Feng
2025-07-04 22:05 ` Benno Lossin
2025-07-04 22:30 ` Boqun Feng
2025-07-04 22:49 ` Benno Lossin
2025-07-04 23:21 ` Boqun Feng
2025-07-04 20:25 ` Boqun Feng
2025-07-04 20:45 ` Benno Lossin
2025-07-04 21:17 ` Boqun Feng
2025-07-04 22:38 ` Benno Lossin
2025-07-04 23:21 ` Boqun Feng
2025-07-05 8:04 ` Benno Lossin
2025-07-05 15:38 ` Boqun Feng
2025-07-05 21:43 ` Benno Lossin
2025-06-26 12:15 ` Andreas Hindborg
2025-06-27 15:01 ` Boqun Feng
2025-06-30 9:52 ` Andreas Hindborg
2025-06-30 14:44 ` Alan Stern
2025-07-01 8:54 ` Andreas Hindborg
2025-07-01 14:50 ` Boqun Feng
2025-07-02 8:33 ` Andreas Hindborg
2025-06-18 16:49 ` [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg operations Boqun Feng
2025-06-21 11:37 ` Gary Guo
2025-06-23 5:23 ` Boqun Feng
2025-06-26 13:12 ` Andreas Hindborg
2025-06-28 3:03 ` Boqun Feng
2025-06-30 10:16 ` Andreas Hindborg
2025-06-30 14:51 ` Alan Stern
2025-06-30 15:12 ` Boqun Feng
2025-06-27 8:58 ` Benno Lossin
2025-06-27 13:53 ` Boqun Feng
2025-06-28 6:12 ` Benno Lossin
2025-06-28 7:31 ` Boqun Feng
2025-06-28 8:00 ` Benno Lossin
2025-06-30 15:24 ` Boqun Feng
2025-06-30 15:27 ` Boqun Feng
2025-06-30 15:50 ` Benno Lossin
2025-06-18 16:49 ` [PATCH v5 06/10] rust: sync: atomic: Add the framework of arithmetic operations Boqun Feng
2025-06-21 11:41 ` Gary Guo
2025-06-26 12:39 ` Andreas Hindborg
2025-06-28 3:04 ` Boqun Feng
2025-06-18 16:49 ` [PATCH v5 07/10] rust: sync: atomic: Add Atomic<u{32,64}> Boqun Feng
2025-06-26 12:47 ` Andreas Hindborg
2025-06-18 16:49 ` [PATCH v5 08/10] rust: sync: atomic: Add Atomic<{usize,isize}> Boqun Feng
2025-06-26 12:49 ` Andreas Hindborg
2025-06-18 16:49 ` [PATCH v5 09/10] rust: sync: atomic: Add Atomic<*mut T> Boqun Feng
2025-06-18 16:49 ` [PATCH v5 10/10] rust: sync: Add memory barriers Boqun Feng
2025-06-26 13:36 ` Andreas Hindborg
2025-06-28 3:42 ` Boqun Feng
2025-06-30 9:54 ` Andreas Hindborg
2025-06-18 20:22 ` [PATCH v5 00/10] LKMM generic atomics in Rust Alice Ryhl
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