Strange results about C++11 memory model (Relaxed ordering)
- by Dancing_bunny
I was testing the example in the memory model of the Anthony Williams's book "C++ Concurrency"
#include<atomic>
#include<thread>
#include<cassert>
std::atomic_bool x,y;
std::atomic_int z;
void write_x_then_y() {
x.store(true, std::memory_order_relaxed);
y.store(true, std::memory_order_relaxed);
}
void read_y_then_x() {
while(!y.load(std::memory_order_relaxed));
if(x.load(std::memory_order_relaxed)) {
++z;
}
}
int main() {
x = false;
y = false;
z = 0;
std::thread a(write_x_then_y);
std::thread b(read_y_then_x);
a.join();
b.join();
assert(z.load()!=0);
}
According to the explanation, relaxed operations on difference variables (here x and y) can be freely reordered. However, I repeated running the problem for more than several days. I never hit the situation that the assertion (assert(z.load()!=0);) fires. I just use the default optimization and compile the code using g++ -std=c++11 -lpthread dataRaceAtomic.cpp
Does anyone actually try it and hit the assertion? Could anyone give me an explanation about my test results? BTW, I also tried the version without using the atomic type, I got the same result. Currently, both programs are running healthily. Thanks.
