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From:   Marco Elver <elver@google.com>
Date:   Thu, 26 Oct 2023 13:16:25 +0200
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Subject: Re: [RFC] rust: types: Add read_once and write_once
To:     Boqun Feng <boqun.feng@gmail.com>
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On Wed, 25 Oct 2023 at 21:54, Boqun Feng <boqun.feng@gmail.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.