Will recursive_mutex deadlock when locked by 2 threads?

Question

std::recursive_mutex won't deadlock when the same thread locks it multiple times.

Will it deadlock when multiple threads lock it multiple times?

Answer 1

Will it deadlock when multiple threads lock it multiple times?

That cannot happen.

By definition a mutex is locked by at most one thread. And a std::recursive_mutex is some kind of mutex. Otherwise it does not comply to its goal of mutual exclusion , since it should be some kind of Mutex concept .

So another thread is not even allowed to lock it a single time. When it does, that other thread stays blocked (till the lock is released - as many times as it was recursively locked).

Answer 2

FYI, here is a simplistic implementation of a recursive mutex:

static const std::thread_id NO_THREAD;

class recursive_mutex {
public:

    void lock() {
        auto me = std::this_thread::get_id();
        if (owner != me) {
            mtx.lock();
            owner = me;
            assert(count == 0);
        }
        count++;
    }

    void unlock() {
        auto me = std::this_thread::get_id();
        assert(owner == me);
        assert(count > 0);
        count--;
        if (count == 0) {
            owner = NO_THREAD;
            mtx.unlock();
        }
    }

private:
    std::mutex      mtx;
    std::thread::id owner;
    unsigned        count{0};
};