notify_all 不唤醒等待线程

Question

DBThread::DBThread() : running_(false)
{

}

DBThread::~DBThread()
{
    if (thread_)
    {
        thread_->join();
    }
}

void DBThread::Init()
{
    thread_ = std::make_shared<std::thread>(std::bind(&DBThread::Run, this));
    std::unique_lock<std::mutex> lock(mutex_);
    cv_.wait(lock, [&] {return running_; });
    std::cout << "Init success";
}

void DBThread::AddTask(std::shared_ptr<Delegate> task)
{
    std::lock_guard<std::mutex> lock(mutex_);
    task_queue_.push(task);
}

void DBThread::Run()
{
    running_ = true;
    cv_.notify_all();
    while (true)
    {
        std::unique_lock<std::mutex> lock(mutex_);
        cv_.wait(lock, [&] {return !task_queue_.empty(); });
        std::cout << "run task" << std::endl;
    }
}

我有两个线程，我们将其命名为 A 和 B，A 调用 Init 并等待 B 完全初始化，即使 running_ 为真，A 有时也会等待。知道为什么会发生这种情况。任何帮助将不胜感激。

Answer 1

std::condition_variable::wait(lock, pred)基本一样

while (!pred()) {
    wait(lock);
}

如果线程将running_ 设置为true 并在检查谓词之间但在调用wait 之前调用notify_all()，则通知将丢失，等待将等待直到另一个通知到来.最简单的修复方法是：

void DBThread::Run()
{
    std::unique_lock<std::mutex> lock(mutex_);
    running_ = true;
    cv_.notify_all();
    while (true)
    {
        cv_.wait(lock, [&] {return !task_queue_.empty(); });
        std::cout << "run task" << std::endl;
    }
}