Re: "Verifying and Optimizing Compact NUMA-Aware Locks on Weak Memory Models"

[Peter Zijlstra]

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

 - some babbling about a missing propagation -- ISTR Linux if stuffed
   full of them, specifically we require stores to 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. Is there actual hardware that
doesn't behave?