But we can do better and further improve the acquire-release semantic of the last post. Why should x be an atomic? There is no reason. That was my first, but incorrect assumption. See why?
A typical misunderstanding in the application of the acquire-release semantic is, to assume, that the acquire operation is waiting for the release operation. So based on this assumption you may think, that x has not to be an atomic variable. So we can further optimize the program.
int x= 0;
std::cout << y.load(std::memory_order_acquire) << " ";
std::cout << x << std::endl;
The program has a data race on x and has therefore undefined behaviour. If y.store(11,std::memory_order_release) (line 12) is executed before y.load(std::memory_order_acquire) (line 16), the acquire-release semantic guarantees, that x= 2000 (line 11) is executed before the reading of x in line 17. But if not. In this case, the reading of x will be executed at the same time as the writing of x. So we have concurrent access on a shared variable, one of them is a write. That's per definition a data race.
The table puts it in a nutshell.
I made this mistake in my presentation "Mulithreading done right?" in Berlin. In Moscow, I did it right. I never claimed, that the C++ memory model is a piece of cake.
Now its time for CppMem. Let's see, what CppMem finds out.
int x= 0;
atomic_int y= 0;
The data race occurs, if one thread is writing x= 2000 and the other thread is reading x. In the graph, we get a dr symbol (data race) on the arrow.
The ultimate step in the process of ongoing optimization is still missing. In the next post, I will use the relaxed semantic.
Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Louis St-Amour, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschläger, Red Trip, Alessandro Pezzato, Evangelos Denaxas, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, Eduardo Velasquez, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Haken Gedek, Bernd Mühlhaus, Challanger, Matthieu Bolt, Stephen Kelley, and Kyle Dean.
Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, Ralf Abramowitsch, and John Nebel.
My special thanks to Embarcadero
My special thanks to PVS-Studio
My new mentoring program "Fundamentals for C++ Professionals" starts in April. Get more information here: https://bit.ly/MentoringProgramModernesCpp.
I'm happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.
Standard Seminars (English/German)
Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.
- Phone: +49 7472 917441
- Mobil:: +49 176 5506 5086
- German Seminar Page: www.ModernesCpp.de
- English Seminar Page: www.ModernesCpp.net