>From 89ab8426f9821b6a8bf86e9d4f7eb596e0cfed73 Mon Sep 17 00:00:00 2001 From: Akira Yokosawa <akiyks@xxxxxxxxx> Date: Wed, 2 Dec 2020 07:35:08 +0900 Subject: [PATCH -perfbook] whymb: Fix description of compiler mischief In this code snippet, the assertion can not fire due to different reasons. When bar() observes b == 1, the smp_mb() assures a == 1. When bar() observes b == 0, the while loop will loop forever and the assertion can't be reached. This was pointed out by Motohiro Kanda in his Japanese translation of perfbook [1]. [1]: https://sites.google.com/site/kandamotohiro/perfbook-d/perfbookappendixf10 Cc: Motohiro Kanda <kanda.motohiro@xxxxxxxxx> Signed-off-by: Akira Yokosawa <akiyks@xxxxxxxxx> --- Hi Paul, (Cc: Motohiro) Looks like this inconsistency in the Answer to Quick Quiz has been Motohiro's translation note for quite a while. (since 2017?) He contributed to perfbook in the past. There is a couple of commits with his Reported-by. Motohiro, do you mind if I submit patches to Paul on behalf of you like this? And I think Paul is likely to do some wordsmithing on top of this patch. ;-) Thanks, Akira -- appendix/whymb/whymemorybarriers.tex | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/appendix/whymb/whymemorybarriers.tex b/appendix/whymb/whymemorybarriers.tex index dff7fafc..301342f3 100644 --- a/appendix/whymb/whymemorybarriers.tex +++ b/appendix/whymb/whymemorybarriers.tex @@ -1152,7 +1152,10 @@ void bar(void) } \end{VerbatimN} - Given this optimization, the assertion could clearly fire. + Given this optimization, the code would behave in a completely + different way than the original intention. + If \co{bar()} observed \qco{b == 0}, the assertion could not + be reached due to the infinite loop. You should use volatile casts or (where available) C++ relaxed atomics to prevent the compiler from optimizing your parallel code into oblivion. -- 2.17.1
