Helgrind possible data race on std::atomic_bool variable

I have similar code in my project:

class foo
{
private:
     mutable std::atomic_bool flag;
     int a, b;
public:
    int GetA() const
    {
        if ( flag )
            //... some сode here
        else
            //...
    }

    int GetB() const
    {
        //...
        flag = true;
        //...
    }
}

The helgrind tool says, that there is possible data race on flag variable, when GetA and GetB methods are called from different threads. I tried to use boost::atomic with similar result. Are any variants to fix it but suppressing?