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Date: Fri, 22 Mar 2024 20:51:03 -0700
From: Boqun Feng <boqun.feng@gmail.com>
To: Kent Overstreet <kent.overstreet@linux.dev>
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Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
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On Fri, Mar 22, 2024 at 11:10:36PM -0400, Kent Overstreet wrote:
> On Fri, Mar 22, 2024 at 07:57:20PM -0700, Boqun Feng wrote:
> > On Fri, Mar 22, 2024 at 10:33:13PM -0400, Kent Overstreet wrote:
> > > 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.
> > 
> > Ah, OK.
> > 
> > > but the new intrinsic would be no different, semantics wise for the
> > > compiler to model, than a "lock orb".
> > 
> > Be ready to be disappointed:
> > 
> > 	https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/is.20atomic.20aliasing.20allowed.3F/near/402078545
> > 	https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/is.20atomic.20aliasing.20allowed.3F/near/402082631
> > 
> > ;-)
> > 
> > In fact, if you get a chance to read the previous discussion links I
> > shared, you will find I was just like you in the beginning: hope we
> > could extend the model to support more kernel code properly. But my
> > overall feeling is that it's either very challenging or lack of
> > motivation to do.
> 
> That's casting - that doesn't work because compiler people hate
> aliasing.
> 
> But intrinsics for e.g.
> __atomic32_read_u8(atomic_u32_t *a, unsigned byte)
> __atomic32_write_u8(atomic_u32_t a*, unsigned byte)
> 

so "byte" here is the byte indexing in the u32? Hmm... I guess that'll
work. But I really don't know whether LLVM/Rust will support such an
intrinsic...

Regards,
Boqun

> should be doable - that's perfectly fine for the compiler to model.
> 
> That would admittedly be ugly to use. But, if Rust ever allowed for
> marking any struct up to word size as atomic (which we want anyways...),
> it could use that under the hood for setting a member variable without
> cmpxchg.