Using condition_variable::notify_all to notify multiple threads

Question

I have been trying to code the dining philosophers as a way to get better with multithreading programming. In my code, I have a condition_variable that stops the thread until all the threads have been created. However, it seems that when I call condition_variable::notify_all to notify that all the threads have been created and to start 'eating', only one thread is notified. For example:

I have a Philosophers class which has these member variables:

static std::condition_variable start;
static std::mutex start_mutex;

And these member function.

static void start_eating() {
    start.notify_all();
}

void dine() {
    signal(SIGINT, ctrl_c_catch);

    std::unique_lock lk{ start_mutex };
    start.wait(lk);

    std::cout << id << "started\n";

    // see end for complete class...

Each thread waits on the condition_variable start and won't continue until I call start_eating(). The problem is that when I call start.notify_all();, only one of the threads gets notified and continues. However, when I change the code to unlock the mutex after waiting, everything runs OK (All the threads continue):

    std::unique_lock lk{ start_mutex };
    start.wait(lk);
    lk.unlock();

I was dont understand what is going on here. Why do I need to unlock the mutex?

The full code:

#include <chrono>
#include <mutex>
#include <vector>
#include <thread>
#include <condition_variable>
#include <atomic>
#include <signal.h>
#include <iostream>
#include <shared_mutex>
#include <ctime>


namespace clk = std::chrono;

const auto EAT_SLEEP_TIME  = clk::milliseconds{1}; // 5 seconds
const auto NUM_SEATS = 5U;

using Fork = std::mutex; // is the fork being used or not

std::mutex cout_mutex;

void ctrl_c_catch(int dummy);

class Philosopher {
    Fork& left;
    Fork& right;
    unsigned id;
    unsigned times_eaten;

    static std::condition_variable start;
    static std::mutex start_mutex;

    static std::atomic_bool end;

public:
    Philosopher(Fork& l, Fork& r, unsigned i) : left{ l }, right{ r }, id{ i }, times_eaten{} {}

    static void start_eating() {
        start.notify_all();
    }

    static void stop_eating() {
        end = true;
    }

    void dine() {
        signal(SIGINT, ctrl_c_catch);

        std::unique_lock lk{ start_mutex };
        start.wait(lk);
        // lk.unlock(); // uncommenting this fixes the issue

        std::cout << id << " started\n";

        while (!end) {
            if (&right < &left) {
                right.lock();
                left.lock();
            } else {
                left.lock();
                right.lock();
            }

            cout_mutex.lock();
            std::clog << id << " got both forks, eating\n";
            cout_mutex.unlock();

            ++times_eaten;

            std::this_thread::sleep_for(EAT_SLEEP_TIME * (rand() % 50));

            right.unlock();
            left.unlock();

            std::this_thread::sleep_for(EAT_SLEEP_TIME * (rand() % 50));
        }

        cout_mutex.lock();
        std::cout << id << " stopped, terminating thread. Eaten " << times_eaten << "\n";
        cout_mutex.unlock();

        delete this;
    }


};

std::atomic_bool Philosopher::end = false;
std::condition_variable Philosopher::start{};
std::mutex Philosopher::start_mutex{};

template <size_t N, typename T = unsigned>
constexpr std::array<T, N> range(T b = 0, T s = 1) {
    std::array<T, N> ret{};

    for (auto& i : ret) {
        i = b;
        b += s;
    }

    return ret;
}

void ctrl_c_catch(int dummy) {
    std::cout << "Caught ctrl-c or stop\nStoping Philosophers\n";
    Philosopher::stop_eating();
    std::this_thread::sleep_for(clk::seconds{5});
    exit(0);
}

int main() {
    srand(time(NULL));

    signal(SIGINT, ctrl_c_catch);

    std::vector<Fork> forks{ NUM_SEATS }; // 5 forks
    std::vector<std::thread> phil; // vector of philosophers

    for (unsigned i : range<NUM_SEATS - 1>()) {
        auto p = new Philosopher{forks[i], forks[i + 1], i};
        phil.emplace_back(&Philosopher::dine, p);
    }
    auto p = new Philosopher{forks[NUM_SEATS - 1], forks[0], NUM_SEATS - 1};
    phil.emplace_back(&Philosopher::dine, p);

    std::clog << "Waiting for 5 seconds\n";
    std::this_thread::sleep_for(clk::seconds{10});

    std::clog << "Starting Philosophers\n Type 'stop' to stop\n";
    Philosopher::start_eating();

    for (auto& t : phil)
        t.detach();

    std::this_thread::sleep_for(clk::seconds{15});
    ctrl_c_catch(0);

    std::string dummy;
    std::cin >> dummy;

    if (dummy == "stop")
        ctrl_c_catch(0);

    return 0;   
}

Answer 1

As explained here, calling std::condition_variable::wait releases the lock, waits, and after waking up, the lock is reacquired. So you need to unlock it manually (or automatically using RAII) to allow other threads to lock it. Condition variables in C++ have similar semantics to non-blocking monitors, so you can read up on that to get a better intuitive understanding. Also, because of spurious unblocking, which is impossible to prevent, you should use the other version of the function, the one that uses a predicate (more info in above link).