linux-kernel - Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

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Date:   Fri, 20 Jan 2023 16:34:37 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

On Sat, Jan 21, 2023 at 01:03:50AM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/21/2023 12:19 AM, Paul E. McKenney wrote:
> > On Fri, Jan 20, 2023 at 11:36:15PM +0100, Jonas Oberhauser wrote:
> > > 
> > > On 1/20/2023 10:37 PM, Paul E. McKenney wrote:
> > > > Just out of curiosity, are you [set] up to run LKMM locally at your end?
> > > I don't know what exactly that means. I generally run it on wetware.
> > > But I sometimes ask Hernan to run Dat3M (on his machine) over all the litmus
> > > tests in your repo to spot any obvious problems with variations I consider.
> > > I don't think Dat3M is feature-complete with herd at the moment, just
> > > unbelievably faster. For example I think it ignores all flags in the cat
> > > files.
> > > Oh, I just remembered that I also installed herd7 recently to make sure that
> > > any patches I might send in satisfy herd7 syntax requirements (I think you
> > > called this diagnostic driven development?), but I haven't used it to really
> > > run anything.
> > > 
> > > Is it too obvious that my words usually aren't backed by cold machine logic?
> > Well, there was this in one of your messages from earlier today: "I'm not
> > going to get it right today, am I?"  And I freely confess that this led
> > me to suspect that you might not have been availing yourself of herd7's
> > opinion before posting.  ;-)
> The main reason I might usually not consult herd7's opinion is that it often
> takes a while to write a test case in a way herd7 accepts and treats as
> intended, but then even so the fact that some tests pass may just give some
> false confidence when some tricky case is being missed.
> So I find the investment/increased confidence ratio to not yet be at the
> right point to do this when communicating somewhat informally on the mailing
> list, which is already taking quite a bit of my time (but at least I'm
> learning a lot during that time about stuff like RCU/SRCU, history of LKMM,
> etc.).
> If I need to be more confident I'll use herd7 to make sure the syntax is
> correct and as a sanity check, and some paper or Coq proofs to be confident
> in the logic.
> 
> If you feel that I'm wasting the lists' time too much by making these kind
> of mistakes, let me know and I'll reconsider.

Not a goal of mine, actually.

The only thing that I will add is that I cheat horribly by creating new
litmus tests by existing ones.  ;-)

							Thanx, Paul