Re: [WIP 0/3] Memory model and atomic API in Rust

[Kent Overstreet]

On Fri, Mar 22, 2024 at 07:26:28PM -0700, Boqun Feng wrote:
> On Fri, Mar 22, 2024 at 10:07:31PM -0400, Kent Overstreet wrote:
> [...]
> > > Boqun already mentioned the "mixing access sizes", which is actually
> > > quite fundamental in the kernel, where we play lots of games with that
> > > (typically around locking, where you find patterns line unlock writing
> > > a zero to a single byte, even though the whole lock data structure is
> > > a word). And sometimes the access size games are very explicit (eg
> > > lib/lockref.c).
> > 
> > I don't think mixing access sizes should be a real barrier. On the read
> 
> Well, it actually is, since mixing access sizes is, guess what,
> an undefined behavior:
> 
> (example in https://doc.rust-lang.org/std/sync/atomic/#memory-model-for-atomic-accesses)
> 
> 	thread::scope(|s| {
> 	    // This is UB: using different-sized atomic accesses to the same data
> 	    s.spawn(|| atomic.store(1, Ordering::Relaxed));
> 	    s.spawn(|| unsafe {
> 		let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic);
> 		differently_sized.store(2, Ordering::Relaxed);
> 	    });
> 	});
> 
> Of course, you can say "I will just ignore the UB", but if you have to
> ignore "compiler rules" to make your code work, why bother use compiler
> builtin in the first place? Being UB means they are NOT guaranteed to
> work.

That's not what I'm proposing - you'd need additional compiler support.
but the new intrinsic would be no different, semantics wise for the
compiler to model, than a "lock orb".