C++ conditional waiting stops execution

July 7, 2015 0 Comments

I have the following class method:

myclass::concurFnc(bool changingVar ) {
{
    static int i = 0;
    ++i;
    std::cout << "Try Waiting: " << i << std::endl;
    std::mutex waitValidMutex;
    std::unique_lock<std::mutex> l(waitValidMutex);
    mNoOtherThreadDiffCompareVal.wait(l, [this, &changingVar]()
    {
        mSemMutex.lock();
        bool result = false;
        if(myclass::staticVar == useCorrectCV)
        {
            std::cout << "Same var : " << changingVar << i << std::endl;
            result = true;
            --mSem;
        } else if(mSem == 1)
        {
            std::cout << "Only one in crit section: " << i << std::endl;
            --mSem;
            myclass::staticVar = changingVar;
            result = true;
        } else
        {
            std::cout << "wait: " << i << std::endl;
            result = false;
        }
        std::cout << "Sem is now " << mSem << std::endl;
        mSemMutex.unlock();
        return result;
    });

    //DO STUFF

    mSemMutex.lock();
    ++mSem;
    std::cout << "In the end sem is: " << mSem << std::endl;
    mSemMutex.unlock();
    mNoOtherThreadDiffCompareVal.notify_all();
}
//class Member Variables
std::mutex mSemMutex;
std::condition_variable mNoOtherThreadDiffCompareVal;

It should allow to enter the //DO STUFF section if changingVar is the same as for all other callers who are currently inside //DO STUFF (or if its the only one)
The function is called concurrent from functions in boost::threads.

Sometimes it happened that the execution stops because it keeps on waiting although mSem is 1 and notify_all() should have been called

Output before execution stops is:

wait: 3248
Sem is now 0
Try Waiting: 3249
wait: 3249
Sem is now 0
In the end sem is: 1
Only one in crit section: 3249
Sem is now 0
Try Waiting: 3250
wait: 3250
Sem is now 0
In the end sem is: 1
Only one in crit section: 3250
Sem is now 0
In the end sem is: 1
Try Waiting: 3251
Same comp val : 0 of: 3251
Sem is now 0
.In the end sem is: 1

Up to now i could not really reproduce it.

Source: c++