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Date:   Thu, 26 Oct 2023 13:16:25 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Gary Guo <gary@garyguo.net>
Cc:     Boqun Feng <boqun.feng@gmail.com>, rust-for-linux@vger.kernel.org,
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        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC] rust: types: Add read_once and write_once
Message-ID: <20231026111625.GK33965@noisy.programming.kicks-ass.net>
References: <20231025195339.1431894-1-boqun.feng@gmail.com>
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 <20231026113610.1425be1b@eugeo>
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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.