linux-kernel - Re: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of LKMM terms

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Date:   Fri, 6 Nov 2020 21:32:14 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, parri.andrea@...il.com,
        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
Subject: Re: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of
 LKMM terms

On Fri, Nov 06, 2020 at 01:04:13PM -0800, Paul E. McKenney wrote:
> On Fri, Nov 06, 2020 at 03:40:08PM -0500, Alan Stern wrote:
> > Is it really true that data dependencies are so easily destroyed?  I 
> > would expect that a true "semantic" dependency (i.e., one where the 
> > value written really does vary according to the value read) would be 
> > rather hard to second guess.
> 
> The usual optimizations apply, for but one example:
> 
> 	r1 = READ_ONCE(x);
> 	WRITE_ONCE(y, (r1 + 1) % MAX_ELEMENTS);
> 
> If MAX_ELEMENTS is 1, so long, data dependency!

Sure, but if MAX_ELEMENTS is 1 then the value written will always be 0 
no matter what value r1 has, so it isn't a semantic dependency.  
Presumably a semantic data dependency would be much more robust.

I wonder if it's worth pointing out this distinction to the reader.

> With pointers, the compiler has fewer optimization opportunities,
> but there are still cases where it can break the dependency.
> Or transform it to a control dependency.

Transforming a data dependency into a control dependency wouldn't make 
any important difference; the hardware would still provide the desired 
ordering.

Alan