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

[Boqun Feng]

On Thu, Oct 26, 2023 at 01:16:25PM +0200, Peter Zijlstra wrote:
> 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.
> 

Maybe we can workaround this limitation by using compiler barriers, i.e.

	compiler_fence(SeqCst);
	load(Relaxed);
	compiler_fence(Acquire);

this is slightly stronger than a volatile atomic.

> > * 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.
> 

Sounds good to me. If the compiler barriers make sense for now, then
we can do:

	pub unsafe fn read_once_usize(ptr: *const usize) -> usize {
		core::sync::atomic::compiler_fence(SeqCst);
		let r = unsafe { *ptr.cast::<AtomicUsize>() }.load(Relaxed);
		core::sync::atomic::compiler_fence(Acquire);
		r
	}

and if the other side (i.e. write) is also atomic (e.g. WRITE_ONCE()),
we don't have data race.

However, there are still cases where data races are ignored in C code,
for example inode::i_state: reads out of locks race with writes inside
locks, since writes are done by plain accesses. Nothing can be done to
fix that from Rust side only, and fixing the C side is a separate topic.

Thoughts?

Regards,
Boqun

> > * 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.
> 
>