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

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Date: Thu, 7 Mar 2024 20:08:46 +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

> > I'd disagree with these premises: certain instructions can and do execute
> > at the same time.
> 
> Can you give an example?

I think I'm starting to see where this is going..., but to address the
question: really any example where the LKMM doesn't know better, say

C test

{}

P0(int *x)
{
	*x = 1;
}

P1(int *x)
{
	*x = 2;
}


> >  FWIW, in the formal model, it is not that difficult to
> > provide examples of "(not F ->xb E) and (not E ->xb F)".
> 
> That's because the xb relation in the formal model does not fully 
> capture our intuitive notion of "executes at the same time" in the 
> informal operational model.
> 
> Also, it's important to distinguish between:
> 
> (1)	Two instructions that are forced (say by a dependency) or known 
> 	(say by an rfe link) to execute in a particular order; versus
> 
> (2)	Two instructions that may execute in either order but do execute
> 	in some particular order during a given run of the program.
> 
> The formal xb relation corresponds more to (1), whereas the informal 
> notion corresponds more to (2).

This appears to be the key observation.  For if, in the operational model,
(not F ->xb E) implies (E ->xb F) then I'll apologize for the noise.  :-)


> > > 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.
> 
> That's why I suggested adding an extra sentence to the paragraph (which 
> you did not quote in your reply).  That sentence gave a direct argument.

Well, I read that sentence but stopped at "These contradictions show that"
for the reason I detailed above.

  Andrea