Re: [RFC] rust: types: Add read_once and write_once

[Peter Zijlstra <peterz@infradead.org>]

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.