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

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Date: Thu, 7 Mar 2024 08:45:32 +0800
From: Kenneth-Lee-2012@...mail.com
To: Andrea Parri <parri.andrea@...il.com>
Cc: Alan Stern <stern@...land.harvard.edu>, linux-kernel@...r.kernel.org,
	paulmck@...nel.org
Subject: Re: Question about PB rule of LKMM

On Wed, Mar 06, 2024 at 08:24:42PM +0100, Andrea Parri wrote:
> Date: Wed, 6 Mar 2024 20:24:42 +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
> 
> > Later on, the file includes this paragraph, which answers the question 
> > you were asking:
> > 
> > ---------------------------------------------------------------------
> > The existence of a pb link from E to F implies that E must execute
> > before F.  To see why, suppose that F executed first.  Then W would
> > have propagated to E's CPU before E executed.  If E was a store, the
> > memory subsystem would then be forced to make E come after W in the
> > coherence order, contradicting the fact that E ->coe W.  If E was a
> > load, the memory subsystem would then be forced to satisfy E's read
> > request with the value stored by W or an even later store,
> > contradicting the fact that E ->fre W.
> > ---------------------------------------------------------------------
> 
> TBF, that just explains (not F ->xb E), or I guess that was the origin
> of the question.

Yes. I thought a link was just an unconditional pattern for the
programmer to match the actual code so the other categories (of links in
the same acyclic rule) cannot route back. But now I understand the ->xx*
relation in the begining or end of the link can also add xx to the rule.

> 
>   Andrea

-- 
			-Kenneth Lee