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

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Message-ID: <CAEXW_YQPSi7RyA=Cz5S753uw4SqBp2v+7CqqE3LN9VQ48q40Zg@mail.gmail.com>
Date:   Sun, 11 Sep 2022 06:20:58 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Hernan Luis Ponce de Leon <hernanl.leon@...wei.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "will@...nel.org" <will@...nel.org>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: "Verifying and Optimizing Compact NUMA-Aware Locks on Weak Memory Models"

On Sat, Sep 10, 2022 at 4:41 PM Hernan Luis Ponce de Leon
<hernanl.leon@...wei.com> wrote:
>
> > You were quoting Jonas here, right?  The email doesn't make this obvious
> > because it doesn't have two levels of "> > " markings.
>
> Yes, I was quoting Jonas.
> It seems my mail client did not format the email correctly and I did not notice.
> Sorry for that.
>
> > In general, _no_ two distinct relations in the LKMM have the same propagation
> > properties.  If wmb always behaved the same way as mb, we wouldn't use two
> > separate words for them.
>
> I understand that relations with different names are intended to be different.
> What I meant was
>         "wmb gives weaker propagation guarantees than mb and because of this, liveness of qspinlock is not guaranteed in LKMM"
>

I wonder if this sort of liveness guarantee (or lack thereof) is
really a problem in practice, where writes will eventually propagate
even though they may not for a bit. Is it possible to write a liveness
test case on any hardware, or is this more in the realms of theory?
Either way, quite intriguing!

Thanks,

 - Joel