Question

我们有一个内部内存管理器，我们使用其中一个产品。内存管理器会覆盖new和delete运算符，并且在单线程应用程序中运行良好。但是，我现在的任务是使其适用于多线程应用程序。根据我的理解，下面的伪代码应该可以工作，但它会挂起，即使使用try_lock()。有什么想法吗？

更新＃1

导致“访问冲突”：

#include <mutex>

std::mutex g_mutex;

/*!
\brief Overrides the Standard C++ new operator
\param size [in] Number of bytes to allocate
*/
void *operator new(size_t size)
{
   g_mutex.lock(); // Access violation exception
   ...   
}

导致线程在旋转中永久挂起：

#include <mutex>

std::mutex g_mutex;
bool g_systemInitiated = false;


/*!
\brief Overrides the Standard C++ new operator
\param size [in] Number of bytes to allocate
*/
void *operator new(size_t size)
{
   if (g_systemInitiated == false) return malloc(size);
   g_mutex.lock(); // Thread hangs forever here. g_mutex.try_lock() also hangs
   ...   
}

int main(int argc, const char* argv[])
{
   // Tell the new() operator that the system has initiated
   g_systemInitiated = true;
   ...
}

更新＃2

递归互斥锁也会导致线程在旋转中永久挂起：

#include <mutex>

std::recursive_mutex g_mutex;
bool g_systemInitiated = false;


/*!
\brief Overrides the Standard C++ new operator
\param size [in] Number of bytes to allocate
*/
void *operator new(size_t size)
{
   if (g_systemInitiated == false) return malloc(size);
   g_mutex.lock(); // Thread hangs forever here. g_mutex.try_lock() also hangs
   ...   
}

int main(int argc, const char* argv[])
{
   // Tell the new() operator that the system has initiated
   g_systemInitiated = true;
   ...
}

更新＃3

Jonathan Wakely建议我应该尝试unique_lock和/或lock_guard，但锁定仍会在旋转中挂起。

unique_lock测试：

#include <mutex>

std::mutex g_mutex;
std::unique_lock<std::mutex> g_lock1(g_mutex, std::defer_lock);
bool g_systemInitiated = false;

/*!
\brief Overrides the Standard C++ new operator
\param size [in] Number of bytes to allocate
*/
void *operator new(size_t size)
{
   if (g_systemInitiated == false) return malloc(size);
   g_lock1.lock(); // Thread hangs forever here the first time it is called
   ...   
}

int main(int argc, const char* argv[])
{
   // Tell the new() operator that the system has initiated
   g_systemInitiated = true;
   ...
}

lock_guard测试：

#include <mutex>

std::recursive_mutex g_mutex;
bool g_systemInitiated = false;


/*!
\brief Overrides the Standard C++ new operator
\param size [in] Number of bytes to allocate
*/
void *operator new(size_t size)
{
   if (g_systemInitiated == false) return malloc(size);
   std::lock_guard<std::mutex> g_lock_guard1(g_mutex); // Thread hangs forever here the first time it is called
   ...   
}

int main(int argc, const char* argv[])
{
   // Tell the new() operator that the system has initiated
   g_systemInitiated = true;
   ...
}

我认为我的问题是锁定时C ++ 11互斥库会调用delete。 delete也被覆盖：

/*!
\brief Overrides the Standard C++ new operator
\param p [in] The pointer to memory to free
*/
void operator delete(void *p)
{
    if (g_systemInitiated == false)
    {
       free(p); 
    }
    else
    {
       std::lock_guard<std::mutex> g_lock_guard1(g_mutex);
       ...
    }
}

这会导致死锁情况，除了在锁定或解锁时不会产生对new或delete的任何调用的锁定时，我看不到任何好的解决方案。

更新＃4

我已经实现了自己的自定义递归互斥锁，它没有调用new或delete，也允许同一个线程进入锁定的块。

#include <thread>

std::thread::id g_lockedByThread;
bool g_isLocked = false;
bool g_systemInitiated = false;

/*!
\brief Overrides the Standard C++ new operator
\param size [in] Number of bytes to allocate
*/
void *operator new(size_t size)
{
   if (g_systemInitiated == false) return malloc(size);

   while (g_isLocked && g_lockedByThread != std::this_thread::get_id());
   g_isLocked = true; // Atomic operation
   g_lockedByThread = std::this_thread::get_id();
   ...   
   g_isLocked = false;
}

/*!
\brief Overrides the Standard C++ new operator
\param p [in] The pointer to memory to free
*/
void operator delete(void *p)
{
    if (g_systemInitiated == false)
    {
       free(p); 
    }
    else
    {
       while (g_isLocked && g_lockedByThread != std::this_thread::get_id());
       g_isLocked = true; // Atomic operation
       g_lockedByThread = std::this_thread::get_id();
       ...   
       g_isLocked = false;
    }
}

int main(int argc, const char* argv[])
{
   // Tell the new() operator that the system has initiated
   g_systemInitiated = true;
   ...
}

更新＃5

尝试了Jonathan Wakely的建议，发现微软实施C ++ 11 Mutexs似乎有些不对劲;如果使用/MTd（多线程调试）编译器标志进行编译，则他的示例会挂起，但如果使用/MDd（多线程调试DLL）编译器标志进行编译，则可以正常工作。正如乔纳森正确地指出std::mutex实现应该是constexpr的。这是我用来测试实现问题的VS 2012 C ++代码：

#include "stdafx.h"

#include <mutex>
#include <iostream>

bool g_systemInitiated = false;
std::mutex g_mutex;

void *operator new(size_t size)
{
    if (g_systemInitiated == false) return malloc(size);
    std::lock_guard<std::mutex> lock(g_mutex);
    std::cout << "Inside new() critical section" << std::endl;
    // <-- Memory manager would be called here, dummy call to malloc() in stead
    return malloc(size);
}

void operator delete(void *p)
{
    if (g_systemInitiated == false) free(p); 
    else
    {
        std::lock_guard<std::mutex> lock(g_mutex);
        std::cout << "Inside delete() critical section" << std::endl;
        // <-- Memory manager would be called here, dummy call to free() in stead
        free(p);
    }
}

int _tmain(int argc, _TCHAR* argv[])
{
    g_systemInitiated = true;

    char *test = new char[100];
    std::cout << "Allocated" << std::endl;
    delete test;
    std::cout << "Deleted" << std::endl;

    return 0;
}

更新＃6

向Microsoft提交错误报告： https://connect.microsoft.com/VisualStudio/feedback/details/776596/std-mutex-not-a-constexpr-with-mtd-compiler-flag#details

Answer 1

默认情况下，互斥锁库使用new，而std :: mutex 不递归（即可重入）。一个鸡蛋问题。

UPDATE 正如下面的评论中指出的那样，使用std :: recursive_mutex可能会有效。但是，对于全局互斥体的静态初始化顺序的经典C ++问题仍然存在，外部访问全局互斥体的危险仍然存在（最好将其置于匿名命名空间内。）

UPDATE 2 您可能过早地将g_systemInitiated切换为true，即在互斥锁有机会完成初始化之前，因此对{{1的“首次通过”调用永远不会发生。要强制执行此操作，您可以尝试使用分配器模块中的初始化函数调用替换main（）中的赋值：

malloc()

Answer 2

除了第一个示例之外，您的所有示例都已损坏，这对于不使用作用域锁定类型来说是非常糟糕的做法。

如果您的编译器或标准库没有损坏，这应该可以工作：

#include <mutex>

std::mutex g_mutex;

void *operator new(size_t size)
{
   std::lock_guard<std::mutex> lock(g_mutex);
   ...   
}

重写新运算符时，std :: mutex锁会挂起

2 个答案: