std::condition_variable::wait 没有释放对 RaspberryPi 的锁定

Question

我在 Ubuntu 上使用 git://github.com/raspberrypi/tools.[=35= 的工具为 RaspberryPi(3) 交叉编译了一个项目].我想使用 condition_variable, but wait_for 的线程同步似乎不会释放提供给它的 mutex 上的锁，如文档中所述。这是在 RaspberryPi 上重现问题的简化代码（有过多的日志记录以更好地可视化问题）：

int main(int argc, const char* args[])
{
    std::condition_variable cv;
    std::mutex m;
    bool ok = false;

    std::cout << "locking in main" << std::endl;
    std::unique_lock<std::mutex> lock(m);
    std::cout << "locked in main" << std::endl;

    std::cout << "starting thread" << std::endl;
    std::thread t([&cv, &m, &ok]() {
        std::cout << "locking in thread" << std::endl;
        std::unique_lock<std::mutex> lock(m);
        std::cout << "locked in thread" << std::endl;
        ok = true;

        std::cout << "unlocking in thread" << std::endl;
        lock.unlock();
        std::cout << "signalling cv" << std::endl;
        cv.notify_one();
    });

    std::cout << "starting wait" << std::endl;
    bool success = cv.wait_for(lock, std::chrono::seconds(10), [&ok]{ return ok; });
    std::cout << "finished waiting: " << success << std::endl;

    std::cout << "unlocking in main" << std::endl;
    lock.unlock();
    std::cout << "joining thread" << std::endl;
    t.join();
    std::cout << "thread joined" << std::endl;

    return 0;
}

这是我在 Raspberry 控制台上使用标准 g++ linux 版本并排比较的输出：

RPi version:                linux version:
locking in main             locking in main
locked in main              locked in main
starting thread             starting thread
starting wait               starting wait
locking in thread           locking in thread
finished waiting: 0         locked in thread
unlocking in main           unlocking in thread
joining thread              signalling cv
locked in thread            finished waiting: 1
unlocking in thread         unlocking in main
signalling cv               joining thread
thread joined               thread joined

这让我相信代码没有问题，但 RaspberryPi 交叉编译或标准库存在问题。我是否遗漏了什么，这个问题有解决方案吗？

编辑： 这个问题首先在依赖于 boost 和 socket.io-client 的更大项目中被发现。我将 main 简化为问题中的那个，但没有删除其余代码。它的编译和链接如下：

[...]
arm-linux-gnueabihf-g++ -std=c++1y -I%SRC_DIR%/rapidjson/include -I%SRC_DIR%/boost_1_65_0/install/include -I%SRC_DIR%/socket.io-client-cpp/build/include -O3 -g -Wall -c -fmessage-length=0 -MMD -MP -MF"main.d" -MT"main.o" -o "main.o" "../main.cpp"
[...]
arm-linux-gnueabihf-g++ -L%SRC_DIR%/boost_1_65_0/install-arm/lib -L%SRC_DIR%/socket.io-client-cpp/build-arm/lib/Release -L%SRC_DIR%/openssl/build-arm/lib -static -pthread -o "main" [...] ./main.o [...] -lsioclient -lboost_system -lssl -lcrypto -ldl

其他目标文件替换为 [...]，socket.io-client 需要 libssl。但是，如果我仅使用 arm-linux-gnueabihf-g++ -pthread -std=c++1y -O3 -g -Wall -o main1 main1.cpp 编译已发布的代码段，它在 RaspberryPi 上也能正常工作，所以这可能不是交叉编译工具的问题。是否有可能依赖项之一对正在发生的事情负责？

Answer 1

试试这个

std::cout << "locking in main" << std::endl;
std::unique_lock<std::mutex> lock(m);
std::cout << "locked in main" << std::endl;

std::cout << "starting thread" << std::endl;
std::thread t([&cv, &m, &ok]() {
    std::cout << "locking in thread" << std::endl;
    std::unique_lock<std::mutex> lock(m);
    std::cout << "locked in thread" << std::endl;
    ok = true;
    std::cout << "unlocking in thread" << std::endl;
    std::cout << "signalling cv" << std::endl;
    cv.notify_one();
});

std::cout << "starting wait" << std::endl;
while(!ok)
    cv.wait(lock);
std::cout << "finished waiting: " << ok << std::endl;

std::cout << "unlocking in main" << std::endl;
lock.unlock();
std::cout << "joining thread" << std::endl;
t.join();
std::cout << "thread joined" << std::endl;

此外，我认为你应该避免在线程内解锁互斥量，因为在作用域结束时，unique_lock 会自动释放锁

Answer 2

看来我的 linker 命令不正确，这（可能）导致 pthread 被静态 linked，由于交叉编译，这是不正确的树莓派系统要求的版本。根据我的问题编辑，我原来的错误（简化）link 命令是这样的：

arm-linux-gnueabihf-g++ -Lsome_dirs -static -pthread -o "main" ./main.o ./some_object_files -lsome_libs

在单个 main.cpp 文件的简单情况下，此类 linking 选项在创建线程时报告错误：terminate called after throwing an instance of 'std::system_error' what(): Enable multithreading to use std::thread: Operation not permitted。它最初让我失望，但实际上是一个有价值的线索。

为了解决这个问题，我不得不将 linker 命令修改为静态 link 库，我需要静态 linked，而 linking pthread 是动态的，像这样：

arm-linux-gnueabihf-g++ -o "main" ./main.o ./some_object_fiels -Lsome_dirs -Wl,-Bstatic -lsome_libs -Wl,-Bdynamic -ldl -pthread

std::condition_variable::wait 没有释放对 RaspberryPi 的锁定

std::condition_variable::wait not releasing lock on RaspberryPi

c++

multithreading

g++

condition-variable

raspberry-pi