如何修复C++线程死锁示例

Question

我设计了一个C++11线程死锁。这是使用两个具有多个线程池的独立函数来实现的。如何修正此示例以避免死锁？我认为这个解决方案与锁程序的顺序一致有关。

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

std::mutex kettle;
std::mutex tap;

#define THREAD_POOL 8

void kettle_tap(){

    std::cout << "Locking kettle in " << std::this_thread::get_id() << std::endl;
    // Lock the mutex kettle by creating and using lock_guard kettle_lock.
    std::lock_guard<std::mutex> kettle_lock(kettle);
    std::cout << "Locked kettle in " << std::this_thread::get_id() << std::endl;

    std::cout << "Locking tap in " << std::this_thread::get_id() << std::endl;
    // Lock the mutex tap by creating and using lock_guard tap_lock.
    std::lock_guard<std::mutex> tap_lock(tap);
    std::cout << "Locked tap in " << std::this_thread::get_id() << std::endl;

    std::cout << "Filling kettle in " << std::this_thread::get_id() << std::endl;
}

void tap_kettle(){

    std::cout << "Locking tap in " << std::this_thread::get_id() << std::endl;
    // Lock the mutex tap by creating and using lock_guard tap_lock.
    std::lock_guard<std::mutex> tap_lock(tap);
    std::cout << "Locked tap in " << std::this_thread::get_id() << std::endl;

    std::cout << "Locking kettle in " << std::this_thread::get_id() << std::endl;
    // Lock the mutex kettle by creating and using lock_guard kettle_lock.
    std::lock_guard<std::mutex> kettle_lock(kettle);
    std::cout << "Locked kettle in " << std::this_thread::get_id() << std::endl;

    std::cout << "Filling kettle in " << std::this_thread::get_id() << std::endl;

}

int main(){

    std::thread pool[THREAD_POOL];

    for (int t = 0; t < THREAD_POOL; t += 2){
        pool[t]   = std::thread(kettle_tap);
        pool[t+1] = std::thread(tap_kettle);
    }

    for (int t = 0; t < THREAD_POOL; ++t){
        pool[t].join();
    }

    std::cout << "Threads are all joined" << std::endl;

    return 0;

}

Answer 1

std::lock(Mutexes...)

在您的示例中，kettle_tap()和tap_kettle()都应该以以下内容开头：

std::lock(tap, kettle);

但是互斥参数的顺序并不重要，因此可以在两个函数之间不同。

锁多互斥 锁定作为参数传递的所有对象，必要时阻塞调用线程。 该函数使用对其成员的未指定的调用序列锁定对象，并锁定、try_lock和解锁，以确保所有参数在返回时都被锁定(而不产生任何死锁)。 如果函数无法锁定所有对象(例如，因为其内部调用之一抛出异常)，则该函数首先在失败之前解锁它成功锁定的所有对象(如果有的话)。

稍后，如果您想将锁的所有权转移到std::lock_guard

std::lock(tap, kettle);
std::lock_guard<std::mutex> kettle_lock(kettle, std::adopt_lock);
std::lock_guard<std::mutex> tap_lock(tap, std::adopt_lock);

Answer 2

你是正确的。通过避免循环等待，死锁可以是防制。在您的示例中，为了避免死锁，请在kettle_lock方法中将tap_lock移动到tap_lock之上。这样你就可以得到一个偏序。