C++ condition variable without mutexes?

Question

Problem

I think I'm misunderstanding the CV-Mutex design pattern because I'm creating a program that seems to not need a mutex, only CV.

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Goal Overview

I am parsing a feed from a website from 2 different accounts. Alice, Bob. The parsing task is slow, so I have two separate threads each dedicated to handling the feeds from Alice and Bob.

I then have a thread that receives messages from the network and assigns the work to either the threadA or threadB, depending on who the update message is for. That way the reader/network thread isn't stalled, and the messages for Alice are in-order and the messages for Bob are in-order, too.

I don't care if Alice thread is a little bit behind Bob thread chronologically, as long as the individual account feeds are in-order.

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Implementation Details

This is very similar to a thread pool, except the threads are essentially locked to a fixed-size array of size 2, and I use the same thread for each feed.

I create a AccountThread class which maintains a queue of JSON messages to be processed as soon as possible within the class. Here is the code for that:

#include <queue>
#include <string>
#include <condition_variable>
#include <mutex>

using namespace std;
class AccountThread {

public:
    AccountThread(const string& name) : name(name) { }

    void add_message(const string& d) {
        this->message_queue.push(d);
        this->cv.notify_all(); // could also do notify_one but whatever
    }

    void run_parsing_loop() {
        while (true) {
            std::unique_lock<std::mutex> mlock(lock_mutex);
            cv.wait(mlock, [&] {
                return this->is_dead || this->message_queue.size() > 0;
            });

            if (this->is_dead) { break; }

            const auto message = this->message_queue.front();
            this->message_queue.pop();

            // Do message parsing...
        }
    }

    void kill_thread() {
        this->is_dead = true;
    }


private:
    const string& name;

    condition_variable cv;
    mutex lock_mutex;
    queue<string> message_queue;

    // To Kill Thread if Needed
    bool is_dead;
};

I can add the main.cpp code, but it's essentially just a reader loop that calls thread.add_message(message) based on what the account name is.

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Question

Why do I need the lock_mutex here? I don't see it's purpose since this class is essentially single-threaded. Is there a better design pattern for this? I feel like if I'm including a variable that I don't really need, such as the mutex then I'm using the wrong design pattern for this task.

I'm just adapting the code from some article I saw online about a threadpool implementation and was curious.

Answer 1

First things first: there's no condition_variable::wait without a mutex. The interface of wait requires a mutex. So regarding

I'm creating a program that seems to not need a mutex, only CV

note that the mutex is needed to protect the condition variable itself. If the notion of how you'd have a data race without the mutex doesn't immediately make sense, check Why do pthreads’ condition variable functions require a mutex.

Secondly there's multiple pain points in the code you provide. Consider this version where the problems are addressed and I'll explain the issues below:

class AccountThread {

public:
    AccountThread(const string& name) : name(name) 
    {
        consumer = std::thread(&AccountThread::run_parsing_loop, this); // 1
    }
    
    ~AccountThread()
    {
        kill_thread(); // 2
        consumer.join();
    }

    void add_message(const string& d) {
        {
            std::lock_guard lok(lock_mutex); // 3
            this->message_queue.push(d);
        }
        this->cv.notify_one();
    }

private:
    void run_parsing_loop() 
    {
        while (!is_dead) {
            std::unique_lock<std::mutex> mlock(lock_mutex);
            cv.wait(mlock, [this] { // 4
                return is_dead || !message_queue.empty();
            });

            if (this->is_dead) { break; }

            std::string message = this->message_queue.front();
            this->message_queue.pop();

            string parsingMsg = name + " is processing " + message + "\n";
            std::cout << parsingMsg;
        }
    }

    void kill_thread() {
        {
            std::lock_guard lock(lock_mutex);
            this->is_dead = true;
        }
        cv.notify_one(); // 5
    }

private:
    string name; // 6

    mutable condition_variable cv; // 7
    mutable mutex lock_mutex;
    std::thread consumer;
    queue<string> message_queue;

    bool is_dead{false}; // 8
};

Top to bottom the problems noted (in the numbered comments are):

1. If you have a worker thread class, like AccountThread, it's easier to get right when the class provides the thread. This way only the relevant interface is exposed and you have better control over the lifetime and workings of the consumer.
2. Case in point, when an AccountThread "dies" the worker should also die. In the example above I fix this dependency by killing the consumer thread inside the destructor.
3. add_message caused a data race in your code. Since you intend to run the parsing loop in a different thread, it's wrong to simply push to the queue without having a critical section.
4. It's cleaner to capture this here, e.g. you probably don't need the reference to mlock captured.
5. kill_thread was not correct. You need to notify the, potentially waiting, consumer thread that a change in state happened. To correctly do that you need to protect the state checked in the predicate with a lock.
6. The initial version with const string &name is probably not something you want. Member const references don't extend the lifetime of temporaries, and the way your constructor is written can leave an instance with dangling state. Even if you do the typical checks, overload the constructor with an r-value reference version, you'll be depending on an external string being alive longer than your AccountThread object. Better use a value member.
7. Remember the M&M rule.
8. You had undefined behavior. The is_alive member was used without being initialized.

Demo

All in all, I think the suggested changes point in the right direction. You can also check an implementation of a Go-like communication channel if you want more insight on how something like the TBB component you mention is implemented. Such a channel (or buffer queue) would simplify implementation to avoid manual usage of mutexes, CVs and alive states:

class AccountThread {
public:
    AccountThread(const string& name) : name(name) {
        consumer = std::thread(&AccountThread::run_parsing_loop, this);
    }
    
    ~AccountThread() {
        kill_thread();
        consumer.join();
    }

    void add_message(const string& d) { _data.push(d); }

private:
    void run_parsing_loop() {
        try {
            while (true) {
                // This pop waits until there's data or the channel is closed.
                auto message = _data.pop();
                // TODO: Implement parsing here
            }
        } catch (...) { 
            // Single exception thrown per thread lifetime
        }
    }

    void kill_thread() { _data.set(yap::BufferBehavior::Closed); }

private:
    string name;
    std::thread consumer;
    yap::BufferQueue<string> _data;
};

Demo2