Re: [PATCH] advsync: Adjust context to herd7 litmus test

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 27, 2017 at 07:18:12AM +0900, Akira Yokosawa wrote:
> On 2017/06/26 09:50:42 -0700, Paul E. McKenney wrote:
> > On Tue, Jun 27, 2017 at 12:04:14AM +0900, Akira Yokosawa wrote:
> >> >From 3db6dc5ae7f71c42f91742064ec38db750f3f25d Mon Sep 17 00:00:00 2001
> >> From: Akira Yokosawa <akiyks@gmail.com>
> >> Date: Mon, 26 Jun 2017 22:33:30 +0900
> >> Subject: [PATCH] advsync: Adjust context to herd7 litmus test
> >>
> >> Commit 96ab6febd94c ("advsync: Convert memory-misordering table to
> >> herd7 litmus test") needs further adjustment.
> >>
> >> Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
> > 
> > Queued and pushed, thank you!
> > 
> > Proofreading the surrounding paragraphs showed some room for improvement,
> > so I have queued the following patch.  Thoughts?
> 
> Now, they are clear and easy enough for me to see the point.
> 
> If you'd like,
> 
> Acked-by: Akira Yokosawa <akiyks@gmail.com>

Applied and pushed, thank you!

							Thanx, Paul

>            Thanks, Akira
> 
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit fb481305ecb2185749586b85dddf38bff3ebd494
> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Date:   Mon Jun 26 09:49:21 2017 -0700
> > 
> >     advsync: Wordsmith memory-barriers intro
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > diff --git a/advsync/memorybarriers.tex b/advsync/memorybarriers.tex
> > index 4a2c334728d3..d758cfb70a8c 100644
> > --- a/advsync/memorybarriers.tex
> > +++ b/advsync/memorybarriers.tex
> > @@ -46,27 +46,24 @@ RCU.
> >  \end{figure}
> >  
> >  Unfortunately, these intuitions break down completely in face of
> > -code that makes direct use of explicit memory barriers for data structures
> > -in shared memory.
> > -For example, the litmus test in
> > +code that fails to use standard mechanisms.
> > +For example, the trivial-seeming litmus test in
> >  Figure~\ref{fig:advsync:Memory Misordering: Store-Buffering Litmus Test}
> > -appears to guarantee that the \co{exists} clause is never validated.
> > -After all, if \nbco{0:r2=0},\footnote{
> > +appears to guarantee that the \co{exists} clause is never satisfied.
> > +After all, if \nbco{0:r2=0}\footnote{
> >  	That is, Thread~\co{P0()}'s instance of local variable \co{r2}
> >  	equals zero.
> >  	See Section~\ref{sec:formal:Anatomy of a Litmus Test}
> > -	for documention of litmus-test structure.}
> > +	for documention of litmus-test nomenclature.}
> >  as shown in the \co{exists} clause, we might hope that Thread~\co{P0()}'s
> >  load from~\co{x1} must have happened before Thread~\co{P1()}'s store to~\co{x1},
> >  which might raise
> >  further hopes that Thread~\co{P1()}'s load from~\co{x0} must happen after
> >  Thread~\co{P0()}'s store to~\co{x0}, so that \nbco{1:r2=2},
> > -which should negate the \co{exits} clause.
> > -The example is symmetric, so similar hopeful reasoning might lead
> > +thus not satisfying the \co{exists} clause.
> > +The example is symmetric, so similar reasoning might lead
> >  us to hope that \nbco{1:r2=0} guarantees that \nbco{0:r2=2}.
> > -Unfortunately, the lack of memory barriers in
> > -Figure~\ref{fig:advsync:Memory Misordering: Store-Buffering Litmus Test}
> > -dashes these hopes.
> > +Unfortunately, the lack of memory barriers dashes these hopes.
> >  Both the compiler and the CPU are within their rights to reorder
> >  the statements within both Thread~\co{P0()} and Thread~\co{P1()},
> >  even on relatively strongly ordered systems such as x86.
> > @@ -77,8 +74,8 @@ which found that the counter-intuitive ordering happened
> >  314 times out of 100,000,000 trials on my x86 laptop.
> >  Oddly enough, the perfectly legal outcome where both loads return the
> >  value 2 occurred less frequently, in this case, only 167 times.
> > -The lesson here is clear: Counterintuitivity does not necessarily
> > -imply lower probability!
> > +The lesson here is clear: Increased counterintuitivity does not necessarily
> > +imply decreased probability!
> >  % Run on June 23, 2017:
> >  % litmus7 -r 1000 -carch X86 C-SB+o-o+o-o.litmus
> >  % Test C-SB+o-o+o-o Allowed
> > 
> > 
>