Issue with lock and condition variable in c++ multithreading

Question

I'm trying to implement a simple example of multithreading in C++ (Windows 10, Visual Studio Express).

I have thread T1 that calculates z = x * x, where x and z are global variables. Thread T2 displays z.

I wanted to use locks and a condition variable.

For some reason the execution gets stuck while T1 un unblocked (after cv.wait, probably in the while loop -- high CPU usage). But this does not occur when I add some code (I tried with cout << "x" << endl;) before the cv.notify_one(); in main(). It's very strange.

Thank you in advance for your help!

Here is the code. I removed the parts I put to kill the threads because I do not get to that part of the algorithm -- the issue is before.

#include "stdafx.h" #include <iostream> #include <thread> #include <mutex> #include <condition_variable> using namespace std; mutex m; condition_variable cv; bool datax = 0, dataz = 0, zPrinted = 0; void T1(const int& x, int& z) { do { unique_lock<mutex> lk(m); cv.wait(lk, [] {return (datax); }); if (datax) { z = x * x; datax = 0; dataz = 1; lk.unlock(); while (dataz) cv.notify_one(); } } while (1); } void T2(const int& z) { do { unique_lock<mutex> lk(m); cv.wait(lk, [] {return (dataz); }); if (dataz) { cout << "z = " << z << endl; dataz = 0; zPrinted = 1; lk.unlock(); while (zPrinted) cv.notify_one(); } } while (1); } int main() { int x, z; char c; thread threadT1(T1, cref(x), ref(z)); thread threadT2(T2, cref(z)); do { unique_lock<mutex> lk(m); cout << "Enter x: "; cin >> x; datax = 1; lk.unlock(); while (datax) { cv.notify_one(); } cv.wait(lk, [] {return zPrinted; }); zPrinted = 0; cout << "Continue? (y/n): "; cin >> c; } while (c == 'y'); return 0; }

Why are you repeatedly notifying the condition variable in both threads? — Mike van Dyke
– Mike van Dyke, Commented May 30, 2018 at 9:01
dataz isn't atomic - and you read/write to it outside the lock. Not a good plan. — UKMonkey
– UKMonkey, Commented May 30, 2018 at 9:06

Mike van Dyke · Accepted Answer · 2018-05-30 09:18:32Z

The problem lies here:

lk.unlock(); while (datax) { cv.notify_one(); } cv.wait(lk, [] {return zPrinted; }); // <-- waiting on unlocked mutex zPrinted = 0;

You are waiting on an unlocked mutex and is undefined behaviour see here under notes.

Leo · Accepted Answer · 2018-06-01 14:14:41Z

Aside from the above indicated problem with waiting on an unlocked mutex:

cv.wait(lk, [] {return zPrinted; });

you have threads that are not rejoined and are running infinite loops. Here's a working version with mods to fix these issues:

#include <iostream> #include <thread> #include <mutex> #include <condition_variable> using namespace std; mutex m; condition_variable cv; bool datax = 0, dataz = 0, zPrinted = 0; bool kill = 0; void T1(const int& x, int& z) { do { unique_lock<mutex> lk(m); cv.wait(lk, [] {return (datax+kill); }); if (kill) return; if (datax) { z = x * x; datax = 0; dataz = 1; lk.unlock(); while (dataz) cv.notify_one(); } } while (1); } void T2(const int& z) { do { unique_lock<mutex> lk(m); cv.wait(lk, [] {return (dataz+kill); }); if (kill) return; if (dataz) { cout << "z = " << z << endl; dataz = 0; zPrinted = 1; lk.unlock(); while (zPrinted) cv.notify_one(); } } while (1); } int main() { int x, z; char c; thread threadT1(T1, cref(x), ref(z)); thread threadT2(T2, cref(z)); do { { unique_lock<mutex> lk(m); cout << "Enter x: "; cin >> x; datax = 1; lk.unlock(); while (datax) cv.notify_one(); } { unique_lock<mutex> lk(m); cv.wait(lk, [] {return zPrinted; }); lk.unlock(); } zPrinted = 0; cout << "Continue? (y/n): "; cin >> c; } while (c == 'y'); kill = 1; cv.notify_all(); threadT1.join(); threadT2.join(); return 0; }

As you see, I introduced a new variable "kill" which, when set to 1, causes both threads to exit their infinite loops, so that they can then be joined.

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