Locking multiple mutexes

For the next few paragraphs, you'll be one of the people who write games. Congratulations, you can play at work!

You're developing a server and you have to write code for exchanging loot between two users:

class user {
    boost::mutex        loot_mutex_;
    std::vector<item_t> loot_;
public:
    // ...

    void exchange_loot(user& u);
};

Each user action could be concurrently processed by different threads on a server, so you have to guard the resources by mutexes. The junior developer tried to deal with the problem, but his solution does not work:

void user::exchange_loot(user& u) {
    // Terribly wrong!!! ABBA deadlocks.
    boost::lock_guard<boost::mutex> l0(loot_mutex_);
    boost::lock_guard<boost::mutex> l1(u.loot_mutex_);
    loot_.swap(u.loot_);
}

The issue in the preceding code is a well-known ABBA deadlock problem...