需要帮助来识别简单多线程代码中的错误

Question

I was trying to solve the problem given here - https://leetcode.com/problems/print-foobar-alternately/ .

I wrote the following code to solve the problem but it exceeds the alloted time limit. I am not able to understand why that is the case. Can someone please point out my mistake ? Also, how can I correct the code given below so that it executes faster while using only while loops to act as a mutex ?

class FooBar {
private:
    int n;
    int state = 0;
public:
    FooBar(int n) {
        this->n = n;
        state = 0;
    }

    void foo(function<void()> printFoo) {

        for (int i = 0; i < n; i++) {

            // printFoo() outputs "foo". Do not change or remove this line.
            while(state == 1);
            printFoo();
            state = 1;
        }
    }

    void bar(function<void()> printBar) {

        for (int i = 0; i < n; i++) {

            // printBar() outputs "bar". Do not change or remove this line.
            while(state == 0);
            printBar();
            state = 0;
        }
    }
};

Answer 1

While loops aren't locks. Locks only allow one thread to pass. In your code if state=0 two threads may print foo one after the another. To solve this problem use a mutex and a unique lock.

 for (int i = 0; i < n; i++) {

            // printFoo() outputs "foo". Do not change or remove this line.
            while(state==1);
            unique_lock<mutex> lck(m1);   //Make mutex m1 as a global mutex
            printFoo();
            state = 1;
        }

Never read the same variable across threads withouth making it atomic or modifying the variable within a lock. In this example since the value state is changing within a shared mutex you don't need to use atomic.