linux-kernel - Re: [PATCH] memory-model: fix cheat sheet typo

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Date:   Thu, 12 Apr 2018 13:21:55 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH] memory-model: fix cheat sheet typo

On Thu, Apr 12, 2018 at 12:18:13PM +0200, Paolo Bonzini wrote:
> On 12/04/2018 11:23, Andrea Parri wrote:
> >>
> >> - smp_store_mb() is missing
> > 
> > Good point. In fact, we could add this to the model as well:
> > following Peter's remark/the generic implementation,
> 
> Good idea.  smp_store_mb() can save some clock cycles in the relatively
> common idiom
> 
> 	write a				write b
> 	read b				read a
> 	if (b)				if (a)
> 	  wake 'em			  we've been woken
> 
> > Yeah, those 'Ordering is cumulative', 'Ordering propagates' could
> > mean different things to different readers... IMO, we may even omit
> > such information; this doc. does not certainly aim for completeness,
> > after all. OTOH, we ought to refrain from making this doc. an excuse
> > to transform (what it is really) high-school maths into some black
> > magic.
> FWIW, what I miss in explanation.txt (and to some extent in the paper)
> is a clear pointer to litmus tests that rely on cumulativity and
> propagation.  In the meanwhile I'll send some patches.  Thanks for the
> feedback, as it also helps validating my understanding of the model.

The litmus test that first comes to my mind when I think of cumulativity
(at least, 'cumulativity' as intended in LKMM) is:

   WRC+pooncerelease+rmbonceonce+Once.litmus

for 'propagation', I could mention:

   IRIW+mbonceonces+OnceOnce.litmus

(both tests are availabe in tools/memory-model/litmus-tests/). It would
be a nice to mention these properties in the test descriptions, indeed.

You might find it useful to also visualize the 'valid' executions (with
the main events/relations) associated to each of these tests; for this,

   $ herd7 -conf linux-kernel.cfg litmus-tests/your-test.litmus \
	-show all -gv

(assuming you have 'gv' installed).

  Andrea


> 
> Thanks,
> 
> Paolo