Ongoing Optimization: A Data Race with CppMem

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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.

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// 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. 

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, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Darshan Mody, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Wolfgang Gärtner,  Louis St-Amour, Stephan Roslen, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Avi Kohn, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, Sudhakar Balagurusamy, lennonli, and Pramod Tikare Muralidhara.

 

Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, and Dendi Suhubdy

 

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