linux-kernel - Re: [PATCH v2] lkmm/docs: Correct ->prop example with additional rfe link

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Date:   Mon, 29 Jul 2019 13:50:44 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jade Alglave <j.alglave@....ac.uk>, linux-arch@...r.kernel.org,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2] lkmm/docs: Correct ->prop example with additional rfe
 link

On Sun, Jul 28, 2019 at 11:35:44AM -0400, Joel Fernandes wrote:
[...]
> > > > > +load of y (rfe link), P2's smp_store_release() ensures that P2's load
> > > > > +of y executes before P2's store to z (second fence), which implies that
> > > > > +that stores to x and y propagate to P2 before the smp_store_release(), which
> > > > > +means that P2's smp_store_release() will propagate stores to x and y to all
> > > > > +CPUs before the store to z propagates (A-cumulative property of this fence).
> > > > > +Finally P0's load of z executes after P2's store to z has propagated to
> > > > > +P0 (rfe link).
> > > > 
> > > > Again, a better change would be simply to replace the two instances of
> > > > "fence" in the original text with "cumul-fence".
> > > 
> > > Ok that's fine. But I still feel the rfe is not a part of the cumul-fence.
> > > The fences have nothing to do with the rfe. Or, I am missing something quite
> > > badly.
> > > 
> > > Boqun, did you understand what Alan is saying?
> > > 
> > 
> > I think 'cumul-fence' that Alan mentioned is not a fence, but a
> > relation, which could be the result of combining a rfe relation and a
> > A-cumulative fence relation. Please see the section "PROPAGATION ORDER
> > RELATION: cumul-fence" or the definition of cumul-fence in
> > linux-kernel.cat.
> > 
> > Did I get you right, Alan? If so, your suggestion is indeed a better
> > change.
> 
> To be frank, I don't think it is better if that's what Alan meant. It is
> better to be explicit about the ->rfe so that the reader walking through the
> example can clearly see the ordering and make sense of it.
> 
> Just saying 'cumul-fence' and then hoping the reader sees the light is quite
> a big assumption and makes the document less readable.
> 

After a bit more rereading of the document, I still think Alan's way is
better ;-)

Please consider the context of paragraph, this is an explanation of an
example, which is about the previous sentence:

	The formal definition of the prop relation involves a coe or
	fre link, followed by an arbitrary number of cumul-fence links,
	ending with an rfe link.

, so using "cumul-fence" actually matches the definition of ->prop.

For the ease of readers, I can think of two ways:

1.	Add a backwards reference to cumul-fence section here, right
	before using its name.

2.	Use "->cumul-fence" notation rather than "cumul-fence" here,
	i.e. add an arrow "->" before the name to call it out that name
	"cumul-fence" here stands for links/relations rather than a
	fence/barrier. Maybe it's better to convert all references to 
	links/relations to the "->" notations in the whole doc.

Thoughts?

Regards,
Boqun

> I mean the fact that you are asking Alan for clarification, means that it is
> not that obvious ;)
> 
> thanks,
> 
>  - Joel
> 
> 

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