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

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Date:   Thu, 26 Oct 2023 13:16:25 +0200
From:   Marco Elver <elver@...gle.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     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>,
        Gary Guo <gary@...yguo.net>,
        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>,
        Peter Zijlstra <peterz@...radead.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>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC] rust: types: Add read_once and write_once

On Wed, 25 Oct 2023 at 21:54, Boqun Feng <boqun.feng@...il.com> wrote:
>
> In theory, `read_volatile` and `write_volatile` in Rust can have UB in
> case of the data races [1]. However, kernel uses volatiles to implement
> READ_ONCE() and WRITE_ONCE(), and expects races on these marked accesses
> don't cause UB. And they are proven to have a lot of usages in kernel.
>
> To close this gap, `read_once` and `write_once` are introduced, they
> have the same semantics as `READ_ONCE` and `WRITE_ONCE` especially
> regarding data races under the assumption that `read_volatile` and
> `write_volatile` have the same behavior as a volatile pointer in C from
> a compiler point of view.
>
> Longer term solution is to work with Rust language side for a better way
> to implement `read_once` and `write_once`. But so far, it should be good
> enough.

One thing you may also want to address is the ability to switch
between READ_ONCE implementations depending on config. For one, arm64
with LTO will promote READ_ONCE() to acquires.