linux-kernel - Re: [RFC] rust: types: Add read_once and write

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Message-ID: <20231026111625.GK33965@noisy.programming.kicks-ass.net>
Date:   Thu, 26 Oct 2023 13:16:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Gary Guo <gary@...yguo.net>
Cc:     Boqun Feng <boqun.feng@...il.com>, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        llvm@...ts.linux.dev, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        kent.overstreet@...il.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        elver@...gle.com, Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC] rust: types: Add read_once and write_once

On Thu, Oct 26, 2023 at 11:36:10AM +0100, Gary Guo wrote:

> There's two reasons that we are using volatile read/write as opposed to
> relaxed atomic:
> * Rust lacks volatile atomics at the moment. Non-volatile atomics are
>   not sufficient because the compiler is allowed (although they
>   currently don't) optimise atomics. If you have two adjacent relaxed
>   loads, they could be merged into one.

Ah yes, that would be problematic, eg, if lifted out of a loop things
could go sideways fast.

> * Atomics only works for integer types determined by the platform. On
>   some 32-bit platforms you wouldn't be able to use 64-bit atomics at
>   all, and on x86 you get less optimal sequence since volatile load is
>   permitted to tear while atomic load needs to use LOCK CMPXCHG8B.

We only grudgingly allowed u64 READ_ONCE() on 32bit platforms because
the fallout was too numerous to fix. Some of them are probably bugs.

Also, I think cmpxchg8b without lock prefix would be sufficient, but
I've got too much of a head-ache to be sure. Worse is that we still
support targets without cmpxchg8b.

It might be interesting to make the Rust side more strict in this regard
and see where/when we run into trouble.

> * Atomics doesn't work for complex structs. Although I am not quite sure
>   of the value of supporting it.

So on the C side we mandate the size is no larger than machine word,
with the exception of the u64 on 32bit thing. We don't mandate strict
integer types because things like pte_t are wrapper types.