undefinedEng

Ongoing Optimization: A Data Race with CppMem

But we can improve and further improve the acquire-release semantics of the last post. Why should x be atomic? There is no reason. That was my first but incorrect assumption. See why?

A typical misunderstanding in applying 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 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 behavior. 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 simultaneously as the writing of x. So we have concurrent access to a shared variable, one of them is a write. That’s, per definition, a data race. 

The table puts it in a nutshell.

undefinedEng

 

 

Rainer D 6 P2 500x500Modernes C++ Mentoring

Be part of my mentoring programs:

  • "Fundamentals for C++ Professionals" (open)
  • "Design Patterns and Architectural Patterns with C++" (open)
  • "C++20: Get the Details" (open)
  • "Concurrency with Modern C++" (starts March 2024)
  • Do you want to stay informed: Subscribe.

     

    I made this mistake in my presentation “Multithreading done right?” in Berlin. In Moscow, I did it right. I never claimed that the C++ memory model is a piece of cake.

    Now it’s 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 writes x= 2000 and the other reads x. The graph shows 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, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, 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, Stephen Kelley, Kyle Dean, Tusar Palauri, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Fütterer, Matthias Grün, Phillip Diekmann, Ben Atakora, Ann Shatoff, Rob North, Bhavith C Achar, Marco Parri Empoli, Philipp Lenk, Charles-Jianye Chen, Keith Jeffery,and Matt Godbolt.

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

    My special thanks to Embarcadero
    My special thanks to PVS-Studio
    My special thanks to Tipi.build 
    My special thanks to Take Up Code
    My special thanks to SHAVEDYAKS

    Seminars

    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.

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

    Online Seminars (German)

    Contact Me

    Modernes C++ Mentoring,

     

     

    0 replies

    Leave a Reply

    Want to join the discussion?
    Feel free to contribute!

    Leave a Reply

    Your email address will not be published. Required fields are marked *