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

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Message-ID: <Y77QbG9lVXX9/B87@rowland.harvard.edu>
Date:   Wed, 11 Jan 2023 10:06:20 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@...wei.com>
Cc:     "paulmck@...nel.org" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "parri.andrea" <parri.andrea@...il.com>, 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 Wed, Jan 11, 2023 at 11:33:33AM +0000, Jonas Oberhauser wrote:
> 
> 
> -----Original Message-----
> From: Paul E. McKenney [mailto:paulmck@...nel.org] 
> Sent: Thursday, January 5, 2023 6:32 PM
> > On Wed, Jan 04, 2023 at 11:13:05AM +0000, Jonas Oberhauser wrote:
> > > -----Original Message-----
> > > From: Alan Stern [mailto:stern@...land.harvard.edu]
> > > Sent: Tuesday, January 3, 2023 7:56 PM
> > > > That all sounds good to me.  However, I wonder if it might be better to use "strong-order" (and similar) for the new relation name instead of "strong-sync".  The idea being that fences are about ordering, not (or not directly) about synchronization.
> > 
> > > I think that is indeed better, thanks. I suppose *-sync might be more appropriate if it *only* included edges between threads.
> 
> > There are quite a few ways to group the relations.  As long as we don't end up oscillating back and forth with too short a frequency, I am good.  ;-)
> 
> Considering how much effort it is to keep the documentation up-to-date 
> even for small changes, I'm extremely oscillation-averse. 
> Interestingly as I go through the documentation while preparing each 
> patch I often find some remarks hinting at the content of the patch, 
> e.g. "fences don't link events on different CPUs" and "rcu-fence is 
> able to link events on different CPUs.  (Perhaps this fact should lead 
> us to say that rcu-fence isn't really a fence at all!)" in the current 
> explanation.txt.
> 
> Following the instructions sent to me by Andrea earlier, right now my 
> plan is to first address the strong ordering in one patch, and then 
> address this perhaps unlucky name of the other "fences" in a second 
> patch. Let me know if this is incorrect, as there is some overlap in 
> that I'll use strong-order right away, and then rename the handful of 
> other fences-but-not-really-at-all to '-order' as well.

Minor snag: There already is an rcu-order relation in the memory model.
Maybe we need a different word from "order".  Or maybe rcu-order should 
be renamed.

Alan