How should I reinitailize or unlock the std::mutex in child process, so that it does not blocked?

I think that the key issue might be about a mutex was locked in the parent process at the time of forking, and then it will appear as locked in the child as well.

Maybe try to use POSIX pthread_atfork with pthread_mutex instead of fork (refer to Linux/UNIX system: pthread_atfork). The mechanism allows to register handlers to prepare and restore mutexes during a fork().

The intent of pthread_atfork() was to provide a mechanism whereby the application (or a library) could ensure that mutexes and other process and thread state would be restored to a consistent state. In practice, this task is generally too difficult to be practicable.

Here’s a simple example:

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void prepare() {
    pthread_mutex_lock(&mutex);
}

void parent() {
    pthread_mutex_unlock(&mutex);
}

void child() {
    pthread_mutex_unlock(&mutex);
}

int main() {
    pthread_atfork(prepare, parent, child);

    if (fork() == 0) {
        // Child process
        pthread_mutex_lock(&mutex);
        std::cout << "Child acquired the lock" << std::endl;
        pthread_mutex_unlock(&mutex);
        std::cout << "Child released the lock" << std::endl;
    } else {
        // Parent process
        pthread_mutex_lock(&mutex);
        std::cout << "Parent acquired the lock" << std::endl;
        // execute some tasks
        pthread_mutex_unlock(&mutex);
        std::cout << "Parent released the lock" << std::endl;
    }
    
    return 0;
}

Hope this is helpful in resolving this issue.