conditional_variable does not trigger when using array of std::mutex - c++

Question

This application is recursive multi-thread detached one. Each thread regenerate
new bunch of threads before it dies.
Option 1 (works) however it's a shared resource hence slows the application down.
Option 2 should remove this bottleneck.
Option 1 works:
std::condition_variable cv;
bool ready = false;
std::mutex mu;
// go triggers the thread's function
void go() {
std::unique_lock<std::mutex> lck( mu );
ready = true;
cv.notify_all();
}
void ThreadFunc ( ...) {
std::unique_lock<std::mutex> lck ( mu );
cv.wait(lck, []{return ready;});
do something useful
}
Option 2 does NOT trigger the thread:
std::array<std::mutex, DUToutputs*MaxGnodes> arrMutex ;
void go ( long m , long Channel )
{
std::unique_lock<std::mutex> lck( arrMutex[m+MaxGnodes*Channel] );
ready = true;
cv.notify_all();
}
void ThreadFunc ( ...) {
std::unique_lock<std::mutex> lck ( arrMutex[Inst+MaxGnodes*Channel] );
while (!ready) cv.wait(lck);
do something useful
}
How can I make option #2 work?

Answer 1

The code in Option 2 contains a so-called data race on the variable ready, because the read and write operations on this variable are no longer synchronized. The behaviour of programs with data races is undefined. You can remove the data race by changing bool ready to std::atomic<bool> ready.
That should already fix the problem in Option 2. However, if you use std::atomic, you can also make other optimizations:
std::atomic<bool> ready{false};
void go(long m, long Channel) {
// no lock required
ready = true;
cv.notify_all();
}
void ThreadFunc( ...) {
std::unique_lock<std::mutex> lck(arrMutex[Inst+MaxGnodes*Channel]);
cv.wait(lck, [] { return ready; });
// do something useful
}