Geek Answers Handbook

C ++ 11 hangs when locking a mutex

Using C ++ 11 std::thread, std::mutexI am writing a simple workflow. However, I had a strange problem with a hang when locking std::mutex, which looks like both threads (main and worker) are trying to block the mutex, but both are locked.

Here is the full code

#include <thread>
#include <condition_variable>
#include <memory>
#include <iostream>
#include <list>
std::condition_variable cv;
std::mutex m;
std::thread t;
bool shouldExit = false;
std::list<int> jobs;

void thread_func()
{
  std::unique_lock<std::mutex> lock(m);
  while (!shouldExit) {
    while (jobs.empty() && !shouldExit) {
      cv.wait(lock);
    }
    // Do some stuff
    if (jobs.empty()) {
      continue;
    }
    // Get a job and do something with it
    if (!lock.owns_lock()) {
      lock.lock();  // <<<< Worker thread hang here
    }
    auto j = std::move(jobs.front());
    jobs.pop_front();
    lock.unlock();
    std::cout << "Do something with job " << j << std::endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
  }
}

int main()
{
  t = std::thread(thread_func);

  for (int i = 1; i < 100; ++i) {
    std::cout << "Push to job " << i << std::endl;
    {
    std::lock_guard<std::mutex> lock(m); // <<<< main thread hang here
    jobs.push_back(i);
    cv.notify_one();
    }
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
  }

  // To wait for thread exit
  shouldExit = true;
  cv.notify_one();
  t.join();
  return 0;
}

I am compiling code using the command below on Ubuntu 14.04

g++ -std=c++11 -g -O0 -pthread -o testthread testthread.cpp

The result of execution is usually as follows:

$ ./testthread
Push to job 1
Do something with job 1
Push to job 2
Do something with job 2
Push to job 3
Push to job 4

The interesting part: when I move one sleep-1ms string code in the main thread in lock_guard, as shown below, the problem disappeared.

  for (int i = 1; i < 100; ++i) {
    std::cout << "Push to job " << i << std::endl;
    {
    std::lock_guard<std::mutex> lock(m);
    jobs.push_back(i);
    cv.notify_one();
    std::this_thread::sleep_for(std::chrono::milliseconds(1)); // Moved into lock_guard
    }
  }

I could not understand why. Could you explain the behavior of the code and what I did wrong?

[] , . , , , .

+4
4

undefined cv.wait lock . :

while (!shouldExit) {
  assert(lock.owns_lock());    // <------ add this
  while (jobs.empty() && !shouldExit) {
    cv.wait(lock);
  }

lib++ wait if !lock.owns_lock(), , .

+2

....

-, . .

void thread_func()
{
  std::unique_lock<std::mutex> lock(m);        // <---- {1}
  while (!shouldExit) {                        // <---- {2}
    while (jobs.empty() && !shouldExit) {      // <---- {3}
      cv.wait(lock);
    }
    // Do some stuff
    if (jobs.empty()) {
      continue;
    }

    if (!lock.owns_lock()) {
      lock.lock();                             // <---- {4}
    }
    auto j = std::move(jobs.front());
    jobs.pop_front();
    lock.unlock();                             // <---- {5}
    std::cout << "Do something with job " << j << std::endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
  }
}

{1} ... , {5}

{2} shouldExit bool. .

{3} - , , . unlock {5}. , .

{4} , , , .

{5} ... .

... {, ... }

lock.lock() thread_func()

....

void thread_func()
{
    .....more code omitted
    ........
    lock.unlock();                            
    std::cout << "Do something with job " << j << std::endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(1));

    lock.lock();   //YOUR NEW LINE
  }
}

.... , , ... continue. , std::condition_variable::wait(), , ...

! !!!... ! std::cout , , ...

​​ std::cout ? (. )

void thread_func()
{
    std::unique_lock<std::mutex> lock(m);
    while (!shouldExit)    // this is redundant, so I removed it in the final code
    {
        while (jobs.empty() && !shouldExit)
        {
            cv.wait(lock, []{ return !jobs.empty(); } );
        }
        // Do some stuff
        auto j = std::move(jobs.front());
        jobs.pop_front();
        //cout is thread-safe but not synchronized
        //std::cout << "Do something with job " << j << std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
    }
}

, , " " std::condition_variable::wait().

...

#include <thread>
#include <condition_variable>
#include <memory>
#include <iostream>
#include <list>
#include <atomic>

std::condition_variable cv;
std::mutex m;
std::mutex mxa;   //for std::cout locking
std::thread t;
std::atomic<bool> shouldExit;
std::list<int> jobs;

namespace detail
{

    std::ostream& safe_print()
    {
        return std::cout;
    }

    template<typename T, typename... Args>
    std::ostream& safe_print(T&& t, Args&&... args)
    {
        std::cout << t;
        return safe_print(std::forward<Args>(args)...);
    }
}

template<typename... Args>
std::ostream& println(Args&&... args)
{
    std::lock_guard<std::mutex> lck(mxa);
    auto&& x = detail::safe_print(std::forward<Args>(args)...);
    std::cout << std::endl;
    return x;
}

void thread_func()
{
    std::unique_lock<std::mutex> lock(m);
    while (jobs.empty() && !shouldExit)
    {
        cv.wait(lock, []{ return !jobs.empty(); } );
    }
    // Do some stuff
    auto j = std::move(jobs.front());
    jobs.pop_front();
    //std::cout << "Do something with job " << j << std::endl;
    println("Do something with job ", j);
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
}

int main()
{
    shouldExit = false;
    //safe_print("This is really funny ", 43, '\n');
    t = std::thread(thread_func);

    for (int i = 1; i < 100; ++i)
    {
        //std::cout << "Push to job " << i << std::endl;
        println("Push to Job ", i);
        {
            std::lock_guard<std::mutex> lock(m); // <<<< main thread doesn't hang here again
            jobs.push_back(i);
            cv.notify_one();
        }
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
    }

    // To wait for thread exit
    shouldExit = true;
    cv.notify_one();
    t.join();
    return 0;
}
+2

:

void thread_func()
{
  while (!shouldExit) 
  {
    int j ;
    {  
      std::unique_lock<std::mutex> lock(m);  // lock object inside while
      while (jobs.empty() && !shouldExit) {
        cv.wait(lock);
      }
      // Do some stuff
      if (jobs.empty()) {
        continue;
      }
      j = jobs.front(); 
      jobs.pop_front();
    } // lock goes out of scope
    std::cout << "Do something with job " << j << std::endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
  }
}

, j , int j ; , .

The main idea of ​​rewriting is to avoid the messy use of the lock function and use it, allowing the constructor / destructor to do the lock job.

Sounds like a job ...

0
source

You have a problem:

   while (jobs.empty() && !shouldExit) {
      cv.wait(lock);
    }
    // Do some stuff
    if (jobs.empty()) {
      continue;
    }

When you wake up, you have a castle. However, by calling continue, you lose the possibility of its release.

0
source

More articles:


All Articles