linux-kernel - Re: [PATCH] tools/memory-model: Make ppo a subrelation of po

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Message-ID: <Y8hS5iq2+vbcYlkT@andrea>
Date:   Wed, 18 Jan 2023 21:22:35 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
        paulmck@...nel.org, will@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org, urezki@...il.com,
        quic_neeraju@...cinc.com, frederic@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tools/memory-model: Make ppo a subrelation of po

> It would be nice here to have a separate term for a potentially 
> cross-CPU fence.
> 
> In fact, why don't we make a concerted effort to straighten out the 
> terminology more fully?  Now seems like a good time to do it.
> 
> To begin with, let's be more careful about the difference between an 
> order-inducing object (an event or pair of events) and the relation of 
> being ordered by such an object.  For instance, given:
> 
> 	A: WRITE_ONCE(x, 1);
> 	B: smp_mb();
> 	C: r1 = READ_ONCE(y);
> 
> then B is an order-inducing object (a memory barrier), and (A,C) is a 
> pair of events ordered by that object.  In general, an order is related 
> to an order-inducing object by:
> 
> 	order = po ; [order-inducing object] ; po
> 
> with suitable modifications for things like smp_store_release where 
> one of the events being ordered _is_ the order-inducing event.
> 
> So for example, we could consistently refer to all order-inducing events 
> as either barriers or fences, and all order-reflecting relations as 
> orders.  This would require widespread changes to the .cat file, but I 
> think it would be worthwhile.
> 
> (Treating "barrier" and "fence" as synonyms seems to be too deeply 
> entrenched to try and fight against.)
> 
> Once that is straightened out, we can distinguish between fences or 
> orders that are weak vs. strong.  And then we can divide up strong 
> fences/orders into single-CPU vs. cross-CPU, if we want to.
> 
> How does that sound?

Sounds like a lot of work, renaming and review, for no clear win
to me.  :-)  But hey, if other are into it...

  Andrea