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

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