Ongoing Optimization: A Data Race with CppMem

18 August 2016
Contents[Show]

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.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
 
// ongoingOptimizationAcquireReleaseBroken.cpp

#include <atomic>
#include <iostream>
#include <thread>

int x= 0;
std::atomic<int> y{0};

void writing(){  
  x= 2000;  
  y.store(11,std::memory_order_release);
}

void reading(){  
  std::cout << y.load(std::memory_order_acquire) << " ";  
  std::cout << x << std::endl;
}

int main(){
  std::thread thread1(writing);
  std::thread thread2(reading);
  thread1.join();
  thread2.join();
};

 

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. 

 

Rainer D 6 P2 540x540Modernes C++ Mentoring

Stay informed about my mentoring programs.

 

 

Subscribe via E-Mail.

The table puts it in a nutshell.

undefinedEng

 

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.

CppMem

 

int main(){
  int x= 0;
  atomic_int y= 0;
  {{{ { 
      x= 2000;
      y.store(11,memory_order_release);
      }
  ||| {
      y.load(memory_order_acquire);
      x;
      }
  }}}
}

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.

raceAcquireRelease

What's next?

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, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschläger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mühlhaus, Matthieu Bolt, Stephen Kelley, Kyle Dean, Tusar Palauri, Dmitry Farberov, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Fütterer, Matthias Grün, Phillip Diekmann, Ben Atakora, and Ann Shatoff.

 

Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, John Nebel, Mipko, Alicja Kaminska, and Slavko Radman.

 

 

My special thanks to Embarcadero CBUIDER STUDIO FINAL ICONS 1024 Small

 

My special thanks to PVS-Studio PVC Logo

 

My special thanks to Tipi.build tipi.build logo

Seminars

I'm happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.

Bookable (Online)

German

Standard Seminars (English/German)

Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.

  • C++ - The Core Language
  • C++ - The Standard Library
  • C++ - Compact
  • C++11 and C++14
  • Concurrency with Modern C++
  • Design Pattern and Architectural Pattern with C++
  • Embedded Programming with Modern C++
  • Generic Programming (Templates) with C++

New

  • Clean Code with Modern C++
  • C++20

Contact Me

Modernes C++,

RainerGrimmDunkelBlauSmall

 

Tags: Ongoing Optimization, CppMem, Atomics, acquire-release semantic

Comments   

0 #1 best songs dubstep 2016-10-12 16:55
Good way of telling, and pleasant paragraph to get data about my presentation subject,
which i am going to present in college.
Quote

Mentoring

Stay Informed about my Mentoring

 

English Books

Course: Modern C++ Concurrency in Practice

Course: C++ Standard Library including C++14 & C++17

Course: Embedded Programming with Modern C++

Course: Generic Programming (Templates)

Course: C++ Fundamentals for Professionals

Interactive Course: The All-in-One Guide to C++20

More Profiles

Contact

Subscribe to the newsletter (+ pdf bundle)

Categories

All tags

Blog archive

Source Code

Most Popular Posts

Visitors

Today 4214

Yesterday 5317

Week 4214

Month 148385

All 11629539

Currently are 253 guests and no members online

Kubik-Rubik Joomla! Extensions

Latest comments