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Date:   Fri, 26 Aug 2022 18:23:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     stern@rowland.harvard.edu, parri.andrea@gmail.com, will@kernel.org,
        boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
        j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com,
        dlustig@nvidia.com, joel@joelfernandes.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: "Verifying and Optimizing Compact NUMA-Aware Locks on Weak
 Memory Models"
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References: <20220826124812.GA3007435@paulmck-ThinkPad-P17-Gen-1>
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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, Hern?n Ponce-de-Le?n, 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?