On Thu, 3 Dec 2020 08:51:43 -0800, Paul E. McKenney wrote: > On Wed, Dec 02, 2020 at 11:50:27PM +0900, Akira Yokosawa wrote: >> >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> > > I took this patch as is, but added a patch on top that (hopefully) > does a better job of explaining the situation. Thank you both!!! Ah, this was your original intention of this Quick Quiz. Yes, this explains the situation much better. I did thought about dropping the smp_mb() in the snippet at first, but that would require me to do a larger rewrite of the answer. Instead, I submitted the minimal patch to attract your attention. ;-) > > Would it also help to add a reference to the discussion of control > dependencies? Yes, I think so. Section 15.3.3 or Section 15.3 as a whole would be a good reference. Thanks, Akira > The straightforward answer given that information is > "control dependencies do not order later loads". > > Motohiro, would you be OK with my mentioning your translation in the > next set of release notes for perfbook? Either way, I much appreciate > your doing this work! > > Thanx, Paul >
