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Date:   Sat, 27 Aug 2022 12:05:33 -0400
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>, 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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On Sat, Aug 27, 2022 at 01:47:48AM +0200, Peter Zijlstra wrote:
> On Fri, Aug 26, 2022 at 01:10:39PM -0400, Alan Stern wrote:
> 
> > >  - 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".
> 
> Linux hard relies on:
> 
> 	CPU0				CPU1
> 
> 	WRITE_ONCE(foo, 1);		while (!READ_ONCE(foo));
> 
> making forward progress.

Indeed yes.  As far as I can tell, this requirement is not explicitly 
mentioned in the LKMM, although it certainly is implicit.  I can't even 
think of a way to express it in a form Herd could verify.

> There were a few 'funny' uarchs that were broken, see for example commit
> a30718868915f.

Ha!  That commit should be a lesson in something, although I'm not sure 
what.  :-)

Alan