Can I read a bool variable in a thread without mutex? [duplicate]

Question

Is there anything wrong in following source code if I don't use mutex?

bool bStop = false;

void thread1_fun()
{
    while (!bStop)
    {
        doSomething();
    }
}

void thread2_fun()
{
    bStop = true;
}

Answer 1

It is undefined behaviour to write to an object in one thread while another thread accesses the object at all.

Unless you specifically inform the compiler that there should be a fence, such as the use of std::atomic, std::mutex et al, all bets are off.

The compiler is within its rights to re-write the code as this:

bool bStop = false;

void thread1_fun()
{
    const bool stopped = bStop;
    // compiler reasons: "because there is no fence, the variable clearly cannot
    // have changed to true, since no-other thread will modify it, since
    // to modify it without a fence would be UB." 
    while (!stopped)  
    {  
        doSomething();
    }
}

void thread2_fun()
{
    // this happens in my thread's view of the world, 
    // but no other thread need see it.
    bStop = true;  
}