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

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Date:   Mon, 29 Aug 2022 05:25:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     stern@...land.harvard.edu, will@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: "Verifying and Optimizing Compact NUMA-Aware Locks on Weak
 Memory Models"

On Mon, Aug 29, 2022 at 04:33:23AM +0200, Andrea Parri 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?
> 
> Section 4 ends with a discussion about certain "spurious" data races.
> Do we have litmus tests with them?  (I could repro with Dartagnan...)

Their Figure 5 clearly shows a data race, but agreed, their claim is
that this race is prevented by other code not in this litmus test.
Me, I currently suspect that the spurious data race might be due to the
failure to guarantee mutual exclusion, though I have not yet read that
paper carefully.  That is scheduled for tomorrow morning.

							Thanx, Paul