当 ||在 std：：atomic 中使用运算符代替 &&？

有一个任务是使 3 个线程始终以特定顺序执行，如下所示：

zero  // prints 0s only
odd   // prints odd numbers    
even  // prints even numbers

每个函数(零、偶数、奇数)分别传递给 3 个线程，因此输出应为：

0102     for n = 2
010203   for n = 3
01020304 for n = 4
and so on

在代码中：

class ZeroEvenOdd {
private:
int n;
std::atomic<int> turn{0};
bool flag = false;
public:
ZeroEvenOdd(int n) {
this->n = n;
}
void zero(std::function<void(int)> printNumber) {
int i = 0;
while (i < n) {
while (turn > 0) {// 1 or 2
std::this_thread::yield();
}
printNumber(0);
turn = !flag ? 1 : 2;
flag = !flag;
++i;
}
}
void even(std::function<void(int)> printNumber) {
int i = 2;
while (i <= n) {
while (turn < 2) {// 0 or 1
std::this_thread::yield();
}
printNumber(i);
turn = 0;
i += 2;
}
}
void odd(std::function<void(int)> printNumber) {
int i = 1;
while (i <= n) {
//while (turn <= 2 && turn != 1) {// 0 or 2 // how does this expression eliminate the race ???
while (turn == 0 || turn == 2) { // this causes race condition
std::this_thread::yield();
}
printNumber(i);
turn = 0;
i += 2;
}
}
};

让我们看一下函数odd：

在内部 while 循环中，我需要检查turn是 0 还是 2：

如果我以这种方式检查：while (turn == 0 || turn == 2) {...}竞争条件出现错误且不完整的输出。

for n = 24
it might be:
010203040506709080110100130120150140170160190180210200230220...(waiting)

我们在这里看到，打印67后哪个是错误的......

但是，如果我以这种方式检查while (turn <= 2 && turn != 1) {...}则不会出现比赛并且输出始终正确。

类似的竞赛出现在其他函数zero和even，当它们的内部 while 循环更改为使用运算符时||。

我知道在表达式中组合原子操作不一定会使整个表达式原子化，但我只是无法弄清楚什么情况会导致此检查while (turn == 0 || turn == 2) {...}???的竞争条件

更新

重现问题的完整代码示例：

#include <iostream>
#include <thread>
#include <atomic>
#include <functional>
class ZeroEvenOdd {
private:
int n;
std::atomic<int> turn{0};
bool flag = false;
public:
ZeroEvenOdd(int n) {
this->n = n;
}
void zero(std::function<void(int)> printNumber) {
int i = 0;
while (i < n) {
while (turn > 0) {// 1 or 2
std::this_thread::yield();
}
printNumber(0);
turn = !flag ? 1 : 2;
flag = !flag;
++i;
}
}
void even(std::function<void(int)> printNumber) {
int i = 2;
while (i <= n) {
while (turn < 2) {// 0 or 1
std::this_thread::yield();
}
printNumber(i);
turn = 0;
i += 2;
}
}
void odd(std::function<void(int)> printNumber) {
int i = 1;
while (i <= n) {
//while (turn <= 2 && turn != 1) {// 0 or 2 // how does this expression eliminate the race ???
while (turn == 0 || turn == 2) { // this causes race condition
std::this_thread::yield();
}
printNumber(i);
turn = 0;
i += 2;
}
}
};  
int main() {
int n = 24;
std::function<void(int)> printNum = [](int x) {
std::cout << x << std::flush;
};
ZeroEvenOdd zeroEvenOdd(n);
std::thread t1(&ZeroEvenOdd::zero, &zeroEvenOdd, printNum);
std::thread t2(&ZeroEvenOdd::even, &zeroEvenOdd, printNum);
std::thread t3(&ZeroEvenOdd::odd,  &zeroEvenOdd, printNum);
t1.join();
t2.join();
t3.join();
return 0;
}

要编译的命令：

g++ -std=c++14 -fsanitize=thread -pthread test.cpp -o test