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@xxxxxxxxx>
>> 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@xxxxxxxxx>
>
> 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@xxxxxxxxx>
Thanks, Akira
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit fb481305ecb2185749586b85dddf38bff3ebd494
> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Date: Mon Jun 26 09:49:21 2017 -0700
>
> advsync: Wordsmith memory-barriers intro
>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> 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
>
>
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