奇怪的shared_ptr行为

Question

我在C++中发现了一个奇怪的行为，std::shared_ptr。 以下示例完美适用于标准指针。

但std::shared_ptr的使用导致此处出现分段故障。 （请参阅下面的回溯）

我知道，从多个线程访问std::shared_ptr并不安全，因此我正在使用原子操作。即使是经典的锁也不会解决问题。

进出口使用gcc version 6.3.0 20170406 (Ubuntu 6.3.0-12ubuntu2)与-Wall -O2 -g和-std=c++17

是否有人知道一个解决方案或为什么代码崩溃？

#include <iostream> 
#include <thread> 
#include <vector> 
#include <mutex> 

template <typename T> 
class list { 
private: 
    struct Node { 
     T value; 
     std::shared_ptr<Node> next; 

     Node() : next(nullptr) {} 
    }; 

    std::shared_ptr<Node> head; 
public: 
    // create dummy-nodes 
    explicit list() : head(std::make_shared<Node>()){ 
     head->next = std::make_shared<Node>(); 
    }; 

    void push_front(T val) { 
     std::shared_ptr<Node> current; 
     std::shared_ptr<Node> next; 
     std::shared_ptr<Node> newNode = std::make_shared<Node>(); 
     newNode->value = val; 

     do { 
      current = std::atomic_load<Node>(&head); 
      next = std::atomic_load<Node>(&current->next); 
      newNode->next = next; 
     } while (!std::atomic_compare_exchange_weak<Node>(&current->next, &next, newNode)); 
    } 
}; 

int main(int argc, char* argv[]) { 

    list<int> ll; 

    std::vector<std::thread> threads; 
    const int thread_count = 8; 
    const int local_count = 200000; 

    std::mutex m; 

    for (int i = 0; i < thread_count; i++) { 
     threads.push_back(std::thread([&ll, i, local_count, &m]() { 
      for (int j = local_count * i; j < local_count * (i + 1); j++) { 
       m.lock(); // optional for testing; doesnt solve the problem 
       ll.push_front(j); 
       m.unlock(); 
      } 

     })); 
    } 

    for (auto& thrd : threads) { 
     thrd.join(); 
    } 

    return 0; 
} 

GDB-回溯：

0x0000555555555d4e in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb0e0) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
150   _M_dispose(); 
(gdb) bt 
#0 0x0000555555555d4e in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb0e0) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
#1 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:662 
#2 std::__shared_ptr<list<int>::Node, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:928 
#3 std::shared_ptr<list<int>::Node>::~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr.h:93 
#4 list<int>::Node::~Node (this=<optimized out>, __in_chrg=<optimized out>) at src/main.cpp:11 
#5 __gnu_cxx::new_allocator<list<int>::Node>::destroy<list<int>::Node> (this=<optimized out>, __p=<optimized out>) at /usr/include/c++/6/ext/new_allocator.h:124 
#6 std::allocator_traits<std::allocator<list<int>::Node> >::destroy<list<int>::Node> (__a=..., __p=<optimized out>) at /usr/include/c++/6/bits/alloc_traits.h:487 
#7 std::_Sp_counted_ptr_inplace<list<int>::Node, std::allocator<list<int>::Node>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:529 
#8 0x0000555555555d51 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb110) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
#9 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:662 
#10 std::__shared_ptr<list<int>::Node, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:928 
#11 std::shared_ptr<list<int>::Node>::~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr.h:93 
#12 list<int>::Node::~Node (this=<optimized out>, __in_chrg=<optimized out>) at src/main.cpp:11 
#13 __gnu_cxx::new_allocator<list<int>::Node>::destroy<list<int>::Node> (this=<optimized out>, __p=<optimized out>) at /usr/include/c++/6/ext/new_allocator.h:124 
#14 std::allocator_traits<std::allocator<list<int>::Node> >::destroy<list<int>::Node> (__a=..., __p=<optimized out>) at /usr/include/c++/6/bits/alloc_traits.h:487 
#15 std::_Sp_counted_ptr_inplace<list<int>::Node, std::allocator<list<int>::Node>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:529 
#16 0x0000555555555d51 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb140) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
#17 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:662 
#18 std::__shared_ptr<list<int>::Node, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:928 
#19 std::shared_ptr<list<int>::Node>::~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr.h:93 
#20 list<int>::Node::~Node (this=<optimized out>, __in_chrg=<optimized out>) at src/main.cpp:11 
#21 __gnu_cxx::new_allocator<list<int>::Node>::destroy<list<int>::Node> (this=<optimized out>, __p=<optimized out>) at /usr/include/c++/6/ext/new_allocator.h:124 
#22 std::allocator_traits<std::allocator<list<int>::Node> >::destroy<list<int>::Node> (__a=..., __p=<optimized out>) at /usr/include/c++/6/bits/alloc_traits.h:487 
#23 std::_Sp_counted_ptr_inplace<list<int>::Node, std::allocator<list<int>::Node>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:529 
#24 0x0000555555555d51 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb170) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
#25 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:662 
#26 std::__shared_ptr<list<int>::Node, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:928 
#27 std::shared_ptr<list<int>::Node>::~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr.h:93 
#28 list<int>::Node::~Node (this=<optimized out>, __in_chrg=<optimized out>) at src/main.cpp:11 
#29 __gnu_cxx::new_allocator<list<int>::Node>::destroy<list<int>::Node> (this=<optimized out>, __p=<optimized out>) at /usr/include/c++/6/ext/new_allocator.h:124 
#30 std::allocator_traits<std::allocator<list<int>::Node> >::destroy<list<int>::Node> (__a=..., __p=<optimized out>) at /usr/include/c++/6/bits/alloc_traits.h:487 
#31 std::_Sp_counted_ptr_inplace<list<int>::Node, std::allocator<list<int>::Node>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:529 
#32 0x0000555555555d51 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb1a0) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
#33 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:662 
#34 std::__shared_ptr<list<int>::Node, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:928 
#35 std::shared_ptr<list<int>::Node>::~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr.h:93 
#36 list<int>::Node::~Node (this=<optimized out>, __in_chrg=<optimized out>) at src/main.cpp:11 
#37 __gnu_cxx::new_allocator<list<int>::Node>::destroy<list<int>::Node> (this=<optimized out>, __p=<optimized out>) at /usr/include/c++/6/ext/new_allocator.h:124 
#38 std::allocator_traits<std::allocator<list<int>::Node> >::destroy<list<int>::Node> (__a=..., __p=<optimized out>) at /usr/include/c++/6/bits/alloc_traits.h:487 
#39 std::_Sp_counted_ptr_inplace<list<int>::Node, std::allocator<list<int>::Node>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:529 
#40 0x0000555555555d51 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb1d0) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
#41 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:662 
#42 std::__shared_ptr<list<int>::Node, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:928 
#43 std::shared_ptr<list<int>::Node>::~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr.h:93 
#44 list<int>::Node::~Node (this=<optimized out>, __in_chrg=<optimized out>) at src/main.cpp:11 
#45 __gnu_cxx::new_allocator<list<int>::Node>::destroy<list<int>::Node> (this=<optimized out>, __p=<optimized out>) at /usr/include/c++/6/ext/new_allocator.h:124 
#46 std::allocator_traits<std::allocator<list<int>::Node> >::destroy<list<int>::Node> (__a=..., __p=<optimized out>) at /usr/include/c++/6/bits/alloc_traits.h:487 
#47 std::_Sp_counted_ptr_inplace<list<int>::Node, std::allocator<list<int>::Node>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:529 
#48 0x0000555555555d51 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release (this=0x7fffd48eb200) at /usr/include/c++/6/bits/shared_ptr_base.h:150 
#49 std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:662 
#50 std::__shared_ptr<list<int>::Node, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:928 
#51 std::shared_ptr<list<int>::Node>::~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/c++/6/bits/shared_ptr.h:93 
#52 list<int>::Node::~Node (this=<optimized out>, __in_chrg=<optimized out>) at src/main.cpp:11 
#53 __gnu_cxx::new_allocator<list<int>::Node>::destroy<list<int>::Node> (this=<optimized out>, __p=<optimized out>) at /usr/include/c++/6/ext/new_allocator.h:124 
#54 std::allocator_traits<std::allocator<list<int>::Node> >::destroy<list<int>::Node> (__a=..., __p=<optimized out>) at /usr/include/c++/6/bits/alloc_traits.h:487 
#55 std::_Sp_counted_ptr_inplace<list<int>::Node, std::allocator<list<int>::Node>, (__gnu_cxx::_Lock_policy)2>::_M_dispose (this=<optimized out>) at /usr/include/c++/6/bits/shared_ptr_base.h:529 

Answer 1

这是一个经典的问题，智能指针和列表节点：节点的析构函数是递归的，它最终耗尽堆栈。这就是堆栈跟踪显示的内容：堆栈溢出。

在CppCon 2016: Herb Sutter “Leak-Freedom in C++... By Default.”他们讨论了这个问题。