Is possible to get a thread-locking mechanism in C++ with a std::atomic_flag?

Question

Using MS Visual C++2012

A class has a member of type std::atomic_flag

class A {
    public:
    ...
    std::atomic_flag lockFlag;
    A () { std::atomic_flag_clear (&lockFlag); } 
};

There is an object of type A

A object;

who can be accessed by two (Boost) threads

void thr1(A* objPtr) { ... }
void thr2(A* objPtr) { ... }

The idea is wait the thread if the object is being accessed by the other thread.

The question is: do it is possible construct such mechanism with an atomic_flag object? Not to say that for the moment, I want some lightweight that a boost::mutex.

By the way the process involved in one of the threads is very long query to a dBase who get many rows, and I only need suspend it in a certain zone of code where the collision occurs (when processing each row) and I can't wait the entire thread to finish join().

I've tryed in each thread some as:

thr1 (A* objPtr) {
    ...
    while (std::atomic_flag_test_and_set_explicit (&objPtr->lockFlag, std::memory_order_acquire)) {
        boost::this_thread::sleep(boost::posix_time::millisec(100));
    }
    ...  /* Zone to portect */

    std::atomic_flag_clear_explicit (&objPtr->lockFlag, std::memory_order_release);
    ...  /* the process continues  */
}

But with no success, because the second thread hangs. In fact, I don't completely understand the mechanism involved in the atomic_flag_test_and_set_explicit function. Neither if such function returns inmediately or can delay until the flag can be locked.

Also it is a mistery to me how to get a lock mechanism with such a function who always set the value, and return the previous value. with no option to only read the actual setting.

Any suggestion are welcome.

Answer 1

By the way the process involved in one of the threads is very long query to a dBase who get many rows, and I only need suspend it in a certain zone of code where the collision occurs (when processing each row) and I can't wait the entire thread to finish join().

Such a zone is known as the critical section. The simplest way to work with a critical section is to lock by mutual exclusion.

The mutex solution suggested is indeed the way to go, unless you can prove that this is a hotspot and the lock contention is a performance problem. Lock-free programming using just atomic and intrinsics is enormously complex and cannot be recommended at this level.

Here's a simple example showing how you could do this (live on http://liveworkspace.org/code/6af945eda5132a5221db823fa6bde49a):

#include <iostream>
#include <thread>
#include <mutex>

struct A
{
    std::mutex mux;
    int x;

    A() : x(0) {}
};

void threadf(A* data)
{
    for(int i=0; i<10; ++i)
    {
        std::lock_guard<std::mutex> lock(data->mux);
        data->x++;
    }
}

int main(int argc, const char *argv[])
{
    A instance;
    auto t1 = std::thread(threadf, &instance);
    auto t2 = std::thread(threadf, &instance);

    t1.join();
    t2.join();

    std::cout << instance.x << std::endl;

    return 0;
}

Answer 2

It looks like you're trying to write a spinlock. Yes, you can do that with std::atomic_flag, but you are better off using std::mutex instead. Don't use atomics unless you really know what you're doing.

Answer 3

To actually answer the question asked: Yes, you can use std::atomic_flag to create a thread locking object called a spinlock.

#include <atomic>

class atomic_lock
{
    public:
        atomic_lock()
            : lock_( ATOMIC_FLAG_INIT )
        {}

        void lock()
        {
            while ( lock_.test_and_set() ) { } // Spin until the lock is acquired.
        }

        void unlock()
        {
            lock_.clear();
        }

    private:
        std::atomic_flag lock_;
};