提升条件变量问题 [英] boost condition variable issue

问题描述

下面的较大程序的最小代码示例将命令从客户端线程发送到asio io_service对象. io_service对象(在Ios类中)正在使用一个线程运行.发送命令后，客户端线程将等待直到 Ios 对象(通过Cmd :: NotifyFinish())通知它已完成.

The following minimal code sample of a larger program sends commands from client threads to an asio io_service object. The io_service object (in the Ios class) is being run with one thread. When the command is sent the client thread waits until it is notified by the Ios object (via Cmd::NotifyFinish()) that it is completed.

该示例似乎可以在Linux Boost 11.46很好的Ubuntu 11.04上运行，但是在Windows 7 boost 1.46上可以断言.

This sample seems to run on Linux Ubuntu 11.04 with boost 1.46 fine but on Windows 7 boost 1.46 it asserts.

我怀疑这与Cmd :: NotifyFinish()中的锁定有关.当我将锁移出嵌套范围时，以便在锁的范围内调用waitConditionVariable_.notify_one()时，它在Windows 7上不会崩溃.但是，boost :: thread文档指出，notify_one()不需要在锁内被调用.

I suspect it is something to do with the lock in Cmd::NotifyFinish(). When I move the lock out of the nested scope so that when waitConditionVariable_.notify_one() is called in the lock's scope it doesn't crash on Windows 7. However, the boost::thread documentation states that notify_one() doesn't need to be called within the lock.

堆栈跟踪(如下)显示它在调用notify_one()时断言.好像cmd对象在调用notify之前就已经消失了.

The stack trace (below) shows it is asserting when notify_one() is called. It is as though the cmd object has disappeared before notify is called...

如何使该线程安全且不声明?

How do I make this thread safe and not assert?

#include <boost/asio.hpp>
#include <boost/thread/thread.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/thread/locks.hpp>
#include <boost/thread/condition_variable.hpp>
#include <boost/bind.hpp>
#include <iostream>

class Cmd
{
public:
    Cmd() :   cnt_(0), waitPred_(false), waiting_(false)
    {
    }
    virtual ~Cmd()
    {
    }
    void BindInfo(int CmdSeq)
    {
        cnt_ = CmdSeq;
    }
    void NotifyFinish()
    {
        // call by service thread...
        {
            boost::mutex::scoped_lock lock(waitMutex_);
            waitPred_ = true;
            if (!waiting_)
            {
                // don't need to notify as isn't waiting
                return;
            }
        }
        waitConditionVariable_.notify_one();
    }
    void Wait()
    {
        // called by worker threads
        boost::mutex::scoped_lock lock(waitMutex_);
        waiting_ = true;
        while (!waitPred_)
            waitConditionVariable_.wait(lock);
    }
    int cnt_;
private:

    boost::mutex waitMutex_;
    boost::condition_variable waitConditionVariable_;
    bool waitPred_;
    bool waiting_;
};


class Ios
{
public:
    Ios() : timer_(ios_), cnt_(0), thread_(boost::bind(&Ios::Start, this))
    {
    }
    void Start()
    {
        timer_.expires_from_now(boost::posix_time::seconds(5));
        timer_.async_wait(boost::bind(&Ios::TimerHandler, this, _1));
        ios_.run();
    }
    void RunCmd(Cmd& C)
    {
        ios_.post(boost::bind(&Ios::RunCmdAsyn, this, boost::ref(C)));
    }

private:
    void RunCmdAsyn(Cmd& C)
    {
        C.BindInfo(cnt_++);
        C.NotifyFinish();
    }
    void TimerHandler(const boost::system::error_code& Ec)
    {
        if (!Ec)
        {
            std::cout << cnt_ << "\n";
            timer_.expires_from_now(boost::posix_time::seconds(5));
            timer_.async_wait(boost::bind(&Ios::TimerHandler, this, _1));
        }
        else
            exit(0);
    }

    boost::asio::io_service ios_;
    boost::asio::deadline_timer timer_;
    int cnt_;
    boost::thread thread_;
};

static Ios ios;

void ThreadFn()
{
    while (1)
    {
        Cmd c;
        ios.RunCmd(c);
        c.Wait();
        //std::cout << c.cnt_ << "\n";
    }
}

int main()
{
    std::cout << "Starting\n";
    boost::thread_group threads;
    const int num = 5;

    for (int i = 0; i < num; i++)
    {
        // Worker threads
        threads.create_thread(ThreadFn);
    }
    threads.join_all();

}

堆栈跟踪

msvcp100d.dll!std::_Debug_message(const wchar_t * message, const wchar_t * file, unsigned int line)  Line 15    C++
iosthread.exe!std::_Vector_const_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > >::_Compat(const std::_Vector_const_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > & _Right)  Line 238 + 0x17 bytes   C++
iosthread.exe!std::_Vector_const_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > >::operator==(const std::_Vector_const_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > & _Right)  Line 203 C++
iosthread.exe!std::_Vector_const_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > >::operator!=(const std::_Vector_const_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > & _Right)  Line 208 + 0xc bytes C++
iosthread.exe!std::_Debug_range2<std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > >(std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _First, std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _Last, const wchar_t * _File, unsigned int _Line, std::random_access_iterator_tag __formal)  Line 715 + 0xc bytes  C++
iosthread.exe!std::_Debug_range<std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > >(std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _First, std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _Last, const wchar_t * _File, unsigned int _Line)  Line 728 + 0x6c bytes    C++
iosthread.exe!std::find_if<std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > >,bool (__cdecl*)(boost::intrusive_ptr<boost::detail::basic_cv_list_entry> const &)>(std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _First, std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _Last, bool (const boost::intrusive_ptr<boost::detail::basic_cv_list_entry> &)* _Pred)  Line 92 + 0x54 bytes    C++
iosthread.exe!std::remove_if<std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > >,bool (__cdecl*)(boost::intrusive_ptr<boost::detail::basic_cv_list_entry> const &)>(std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _First, std::_Vector_iterator<std::_Vector_val<boost::intrusive_ptr<boost::detail::basic_cv_list_entry>,std::allocator<boost::intrusive_ptr<boost::detail::basic_cv_list_entry> > > > _Last, bool (const boost::intrusive_ptr<boost::detail::basic_cv_list_entry> &)* _Pred)  Line 1848 + 0x58 bytes    C++
iosthread.exe!boost::detail::basic_condition_variable::notify_one()  Line 267 + 0xb4 bytes  C++
iosthread.exe!Cmd::NotifyFinish()  Line 41  C++

推荐答案

问题是条件变量是由客户端线程创建的Cmd对象的成员，并且在等待时被该客户端线程破坏了完成.

The problem is that the condition variable is a member of the Cmd object that is created by the client thread and is destroyed by that client thread when the wait is completed.

所以您的比赛条件是:

在服务线程上调用
• boost::condition_variable::notify_one()
• 取消阻止正在等待该条件变量的客户端线程
• 然后，客户端线程可以在调用notify_one时销毁服务线程仍在使用的条件变量.

• boost::condition_variable::notify_one() is called on the 'service thread'
• that unblocks the client thread that's waiting on that condition variable
• the client thread can then destroy the condition variable that the service thread is still working with in its call to notify_one.

因此，我认为您的观察结果好像cmd对象已在调用notify之前消失了".除了Cmd对象在调用notify_one()之前没有消失之外，它在notify_one()工作时消失了.您还需要注意的是，"boost::thread文档指出notify_one()不需要在锁内被调用"是正确的，但这并不意味着条件变量可以在notify_one()返回之前被销毁.

So your observation that it's "as though the cmd object has disappeared before notify is called" is pretty much exactly what's happened, I think. Except that the Cmd object didn't disappear before notify_one() is called, it disappeared while notify_one() was doing its work. Your other note that "the boost::thread documentation states that notify_one() doesn't need to be called within the lock" is true, but that doesn't mean that the condition variable can be destroyed before notify_one() has returned.

您需要管理Cmd对象的生存期，以便服务线程在销毁它之前就已使用它-在调用notify_one()的同时保存Cmd对象中的互斥锁是一种方法(正如您所注意到的).或者，您可以将条件变量从Cmd对象中拉出，以便其生存期独立于Cmd对象(也许shared_ptr<>可以提供帮助).

You need to manage the lifetime of the Cmd object so that the service thread is done using it before it gets destroyed - holding the mutex that's in the Cmd object while notify_one() is called is one way to do that (as you've noticed). Or you can pull the condition variable out of the Cmd object so that its lifetime is independent of the Cmd object (maybe shared_ptr<> can help with that).

此外，请注意，我相信Cmd类的waiting_成员是多余的-当条件变量上没有任何服务员时，您可以调用notify_one()或notify_all()-它已经在检查对您来说(我认为这没有什么害处，只是它的复杂性不需要在Cmd类中).

Also, note that I believe that the waiting_ member of the Cmd class is superfluous - you can call notify_one() or notify_all() when there are no waiters on a condition variable - it's already doing the checking for that for you (I don't think it's hurting anything, just that it's complexity that doesn't need to be in the Cmd class).

这篇关于提升条件变量问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！