Implementing boost::barrier in C++11 - multithreading

Question

I've been trying to get a project rid of every boost reference and switch to pure C++11.
At one point, thread workers are created which wait for a barrier to give the 'go' command, do the work (spread through the N threads) and synchronize when all of them finish. The basic idea is that the main loop gives the go order (boost::barrier .wait()) and waits for the result with the same function.
I had implemented in a different project a custom made Barrier based on the Boost version and everything worked perfectly. Implementation is as follows:
Barrier.h:
class Barrier {
public:
Barrier(unsigned int n);
void Wait(void);
private:
std::mutex counterMutex;
std::mutex waitMutex;
unsigned int expectedN;
unsigned int currentN;
};
Barrier.cpp
Barrier::Barrier(unsigned int n) {
expectedN = n;
currentN = expectedN;
}
void Barrier::Wait(void) {
counterMutex.lock();
// If we're the first thread, we want an extra lock at our disposal
if (currentN == expectedN) {
waitMutex.lock();
}
// Decrease thread counter
--currentN;
if (currentN == 0) {
currentN = expectedN;
waitMutex.unlock();
currentN = expectedN;
counterMutex.unlock();
} else {
counterMutex.unlock();
waitMutex.lock();
waitMutex.unlock();
}
}
This code has been used on iOS and Android's NDK without any problems, but when trying it on a Visual Studio 2013 project it seems only a thread which locked a mutex can unlock it (assertion: unlock of unowned mutex).
Is there any non-spinning (blocking, such as this one) version of barrier that I can use that works for C++11? I've only been able to find barriers which used busy-waiting which is something I would like to prevent (unless there is really no reason for it).

Answer 1

class Barrier {
public:
explicit Barrier(std::size_t iCount) :
mThreshold(iCount),
mCount(iCount),
mGeneration(0) {
}
void Wait() {
std::unique_lock<std::mutex> lLock{mMutex};
auto lGen = mGeneration;
if (!--mCount) {
mGeneration++;
mCount = mThreshold;
mCond.notify_all();
} else {
mCond.wait(lLock, [this, lGen] { return lGen != mGeneration; });
}
}
private:
std::mutex mMutex;
std::condition_variable mCond;
std::size_t mThreshold;
std::size_t mCount;
std::size_t mGeneration;
};

Answer 2

Use a std::condition_variable instead of a std::mutex to block all threads until the last one reaches the barrier.
class Barrier
{
private:
std::mutex _mutex;
std::condition_variable _cv;
std::size_t _count;
public:
explicit Barrier(std::size_t count) : _count(count) { }
void Wait()
{
std::unique_lock<std::mutex> lock(_mutex);
if (--_count == 0) {
_cv.notify_all();
} else {
_cv.wait(lock, [this] { return _count == 0; });
}
}
};

Answer 3

Here's my version of the accepted answer above with Auto reset behavior for repetitive use; this was achieved by counting up and down alternately.
/**
* #brief Represents a CPU thread barrier
* #note The barrier automatically resets after all threads are synced
*/
class Barrier
{
private:
std::mutex m_mutex;
std::condition_variable m_cv;
size_t m_count;
const size_t m_initial;
enum State : unsigned char {
Up, Down
};
State m_state;
public:
explicit Barrier(std::size_t count) : m_count{ count }, m_initial{ count }, m_state{ State::Down } { }
/// Blocks until all N threads reach here
void Sync()
{
std::unique_lock<std::mutex> lock{ m_mutex };
if (m_state == State::Down)
{
// Counting down the number of syncing threads
if (--m_count == 0) {
m_state = State::Up;
m_cv.notify_all();
}
else {
m_cv.wait(lock, [this] { return m_state == State::Up; });
}
}
else // (m_state == State::Up)
{
// Counting back up for Auto reset
if (++m_count == m_initial) {
m_state = State::Down;
m_cv.notify_all();
}
else {
m_cv.wait(lock, [this] { return m_state == State::Down; });
}
}
}
};

Answer 4

Seem all above answers don't work in the case the barrier is placed too near
Example: Each thread run the while loop look like this:
while (true)
{
threadBarrier->Synch();
// do heavy computation
threadBarrier->Synch();
// small external calculations like timing, loop count, etc, ...
}
And here is the solution using STL:
class ThreadBarrier
{
public:
int m_threadCount = 0;
int m_currentThreadCount = 0;
std::mutex m_mutex;
std::condition_variable m_cv;
public:
inline ThreadBarrier(int threadCount)
{
m_threadCount = threadCount;
};
public:
inline void Synch()
{
bool wait = false;
m_mutex.lock();
m_currentThreadCount = (m_currentThreadCount + 1) % m_threadCount;
wait = (m_currentThreadCount != 0);
m_mutex.unlock();
if (wait)
{
std::unique_lock<std::mutex> lk(m_mutex);
m_cv.wait(lk);
}
else
{
m_cv.notify_all();
}
};
};
And the solution for Windows:
class ThreadBarrier
{
public:
SYNCHRONIZATION_BARRIER m_barrier;
public:
inline ThreadBarrier(int threadCount)
{
InitializeSynchronizationBarrier(
&m_barrier,
threadCount,
8000);
};
public:
inline void Synch()
{
EnterSynchronizationBarrier(
&m_barrier,
0);
};
};