层级锁笔记

注意看test_hierarchy_lock函数：如果thread t2的不注释，就会报错。

这是因为层级锁 更强调单个线程内上锁的顺序。
线程t2已经获取了hmtx2，再试图获取hmtx1就会因为违反层级顺序而抛出异常。

#include <mutex>
#include <thread>
//层级锁
class hierarchical_mutex {
public:explicit hierarchical_mutex(unsigned long value) :_hierarchy_value(value),_previous_hierarchy_value(0) {}hierarchical_mutex(const hierarchical_mutex&) = delete;hierarchical_mutex& operator=(const hierarchical_mutex&) = delete;void lock() {check_for_hierarchy_violation(); // 必须_this_thread_hierarchy_value >= 待获取锁的层级值 当前层级值，才继续_internal_mutex.lock();update_hierarchy_value(); // _this_thread_hierarchy_value 变成 待获取锁的层级值}void unlock() {if (_this_thread_hierarchy_value != _hierarchy_value) {throw std::logic_error("mutex hierarchy violated");}_this_thread_hierarchy_value = _previous_hierarchy_value;_internal_mutex.unlock();}bool try_lock() {check_for_hierarchy_violation();if (!_internal_mutex.try_lock()) {return false;}update_hierarchy_value();return true;}
private:std::mutex  _internal_mutex;//当前层级值unsigned long const _hierarchy_value;//上一次层级值unsigned long _previous_hierarchy_value;//本线程记录的层级值static thread_local  unsigned long  _this_thread_hierarchy_value;void check_for_hierarchy_violation() {if (_this_thread_hierarchy_value <= _hierarchy_value) {throw  std::logic_error("mutex  hierarchy violated");}}void  update_hierarchy_value() {_previous_hierarchy_value = _this_thread_hierarchy_value;_this_thread_hierarchy_value = _hierarchy_value;}
};thread_local unsigned long hierarchical_mutex::_this_thread_hierarchy_value(ULONG_MAX);void test_hierarchy_lock() {hierarchical_mutex  hmtx1(1000);hierarchical_mutex  hmtx2(500);std::thread t1([&hmtx1, &hmtx2]() {std::this_thread::sleep_for(std::chrono::milliseconds(500));hmtx1.lock();hmtx2.lock();hmtx2.unlock();hmtx1.unlock();});std::thread t2([&hmtx1, &hmtx2]() {hmtx2.lock();
//		hmtx1.lock();
//		hmtx1.unlock();hmtx2.unlock();});t1.join();t2.join();
}int main(){test_hierarchy_lock();return 0;
}