linux-kernel - Re: Question about PB rule of LKMM

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Date: Thu, 7 Mar 2024 19:18:46 +0100
From: Andrea Parri <parri.andrea@...il.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: Kenneth-Lee-2012@...mail.com, linux-kernel@...r.kernel.org,
	paulmck@...nel.org
Subject: Re: Question about PB rule of LKMM

> So I guess you're talking about the second, intuitive meaning.  That's 
> very simple to explain.  Since every instruction executes at _some_ 
> time, and since we can safely assume that no two instructions execute at 
> exactly the _same_ time, if F doesn't execute before E then E must 
> execute before F.  Or using your terms, (not F ->xb E) implies (E ->xb 
> F).  Would that answer the original question satisfactorily?

I'd disagree with these premises: certain instructions can and do execute
at the same time.  FWIW, in the formal model, it is not that difficult to
provide examples of "(not F ->xb E) and (not E ->xb F)".


> The new text says the same thing as the original, just in a more 
> condensed way.  It skips the detailed explanation of why E must execute 
> before W propagates to E's CPU, merely saying that it is because "W is 
> coherence-later than E".  I'm not sure this is an improvement; the 
> reader might want to know exactly how this reasoning goes.

The current text relies on an argument by contradiction.  A contradiction
is reached by "forcing" (F ->xb E), hence all it can be concluded is that
(not F ->xb E).  Again, AFAICS, this doesn't match the claim in the text.

  Andrea