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SubjectRe: "Verifying and Optimizing Compact NUMA-Aware Locks on Weak Memory Models"
On Fri, Aug 26, 2022 at 06:23:24PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 26, 2022 at 05:48:12AM -0700, Paul E. McKenney wrote:
> > Hello!
> >
> > I have not yet done more than glance at this one, but figured I should
> > send it along sooner rather than later.
> >
> > "Verifying and Optimizing Compact NUMA-Aware Locks on Weak
> > Memory Models", Antonio Paolillo, Hernán Ponce-de-León, Thomas
> > Haas, Diogo Behrens, Rafael Chehab, Ming Fu, and Roland Meyer.
> > https://arxiv.org/abs/2111.15240
> >
> > The claim is that the queued spinlocks implementation with CNA violates
> > LKMM but actually works on all architectures having a formal hardware
> > memory model.
> >
> > Thoughts?
>
> So the paper mentions the following defects:
>
> - LKMM doesn't carry a release-acquire chain across a relaxed op

That's right, although I'm not so sure this should be considered a
defect...

> - some babbling about a missing propagation -- ISTR Linux if stuffed
> full of them, specifically we require stores to auto propagate
> without help from barriers

Not a missing propagation; a late one.

Don't understand what you mean by "auto propagate without help from
barriers".

> - some handoff that is CNA specific and I've not looked too hard at
> presently.
>
>
> I think we should address that first one in LKMM, it seems very weird to
> me a RmW would break the chain like that.

An explicitly relaxed RMW (atomic_cmpxchg_relaxed(), to be precise).

If the authors wanted to keep the release-acquire chain intact, why not
use a cmpxchg version that has release semantics instead of going out of
their way to use a relaxed version?

To put it another way, RMW accesses and release-acquire accesses are
unrelated concepts. You can have one without the other (in principle,
anyway). So a relaxed RMW is just as capable of breaking a
release-acquire chain as any other relaxed operation is.

> Is there actual hardware that
> doesn't behave?

Not as far as I know, although that isn't very far. Certainly an
other-multicopy-atomic architecture would make the litmus test succeed.
But the LKMM does not require other-multicopy-atomicity.

Alan

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