为什么这个使用boost的并发Hasmap不起作用

我有以下并发哈希映射：

template<typename Key, typename Value>
class ConcurrentMap
{
public:
    ConcurrentMap() {};
    bool has(Key k)
    {
        boost::shared_lock<boost::shared_mutex> lock(schemaAccess);
        return m.find(k) != m.end();
    }
    void set(Key k, Value v)
    {
        boost::shared_lock<boost::shared_mutex> lock(schemaAccess);
        // set k, v
        if(m.find(k) != m.end())
        {
            boost::upgrade_lock<boost::shared_mutex> valueLock(*valueAccess[k]);
            boost::upgrade_to_unique_lock<boost::shared_mutex> valueUniqueLock(valueLock);
            m.at(k) = v;
        }
        // insert k, v
        else
        {
            lock.unlock();
            boost::upgrade_lock<boost::shared_mutex> schemaLock(schemaAccess);
            boost::upgrade_to_unique_lock<boost::shared_mutex> schemaUniqueLock(schemaLock);
            boost::shared_ptr<boost::shared_mutex> mtx = boost::make_shared<boost::shared_mutex>();
            valueAccess.insert(std::pair<Key, boost::shared_ptr<boost::shared_mutex> >(k, mtx));
            m.insert(std::pair<Key,Value>(k,v));
        }
    }
    Value get(Key k)
    {
        boost::shared_lock<boost::shared_mutex> lock(schemaAccess);
        return m.at(k);
    }
    bool get(Key k, Value& v)
    {
        boost::shared_lock<boost::shared_mutex> lock(schemaAccess);
        if(m.find(k) != m.end()){
            v = m.at(k);
            return true;
        }
        return false;
    }

private:
    std::map<Key, Value> m;
    std::map<Key, boost::shared_ptr<boost::shared_mutex> > valueAccess;
    boost::shared_mutex schemaAccess;
    //http://stackoverflow.com/questions/36468270/how-to-use-a-boostmutex-as-the-mapped-type-in-stdmap/36469809?noredirect=1#comment60552035_36469809
};

以及以下用法：

ConcurrentMap<int, cv::Mat> mapper;
void worker1(){
    int counter = 0;
    while(1){
        boost::this_thread::sleep(boost::posix_time::milliseconds(5));
        cv::Mat img = cv::Mat(1920,1080, CV_64F);
        cout << "w1" << counter++ << endl;
        mapper.set(0,img);
    }
}
void worker2(){
    int counter = 0;
    while(1){
        boost::this_thread::sleep(boost::posix_time::milliseconds(5));
        cout << "w2" << counter++ << endl;
        cv::Mat img;
        if(!mapper.get(0, img)) continue ;
        cout << img.rows << endl;
        cout << "done" << endl;
    }
}
int main(int argc, char **argv)
{
    ros::init(argc, argv, "loading_area_concat");

    boost::thread worker1_;
    boost::thread worker2_;
    worker1_ = boost::thread(worker1);
    worker2_ = boost::thread(worker2);
    ros::spin();
    return 0;
}
*** Error in `/home/raaj/catkin_ac2/devel/lib/loading_area/loading_area_concat': free(): invalid pointer: 0x00007fb71c000078 ***

然而，我的代码在第二个线程的"get"部分不断崩溃。如果我评论出"得到"，它就起作用了。要么是这样，要么如果我用下面的代码完全锁定get代码，它就会起作用：

    boost::upgrade_lock<boost::shared_mutex> schemaLock(schemaAccess);
    boost::upgrade_to_unique_lock<boost::shared_mutex> schemaUniqueLock(schemaLock);

我似乎不明白为什么会发生这种事。当我向哈希图中的密钥写入时，我正在锁定它，当我读取它时，写入程序不应该进来破坏内存，是吗？