多线程操作全局变量,必须考虑同步问题,否则可能出现数据不一致, 甚至触发coredump.
前段时间, 遇到一个多线程操作了全局的vector的问题, 程序崩了。场景是这样的:某全局配置参数保存在一个vector中,需要定时更新(更新线程), 另外的工作线程去读取配置。 这种场景是非常普遍的。
在该场景中,程序没有枷锁,概率coredump, 实际情况是,服务跑了一段时间后,必然coredump. 很显然, 更新线程执行clear,然后在push_back操作时, 会导致工作线程的vector迭代器失效, 内存错误。
本文中, 我从实例和代码的层面来说一下, 在C++ STL中, vector并不是线程安全的, 大家使用的时候, 要多加小心。 为了简便起见, 不采用上面的原场景, 而是仅仅以push_back为例:
来看一段程序:
#include <pthread.h>
#include <unistd.h>
#include <iostream>
#include <vector>
#define N 2
using namespace std;
vector<int> g_v;
pthread_mutex_t mutex;
void* fun(void *p)
{
for(int i = 0; i < 100000; i++)
{
//pthread_mutex_lock(&mutex);
g_v.push_back(i);
//pthread_mutex_unlock(&mutex);
}
return NULL;
}
int main()
{
pthread_t threads[ N];
pthread_mutex_init(&mutex, NULL);
for(int i = 0; i < N; i++)
{
pthread_create(&threads[i], NULL, fun, NULL);
}
for(int i = 0; i < N; i++)
{
pthread_join(threads[i],NULL);
}
cout << "ok" << endl;
return 0;
}
编译: g++ test.cpp -lpthread -g
运行3次:
taoge:~> ./a.out
ok
taoge:~> ./a.out
Segmentation fault (core dumped)
taoge:~> ./a.out
ok
可见, 程序概率core dump. 来调试一下:
taoge:~> gdb a.out core.9775
GNU gdb 6.6
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i586-suse-linux"...
Using host libthread_db library "/lib/libthread_db.so.1".
warning: Can‘t read pathname for load map: Input/output error.
Reading symbols from /lib/libonion.so...done.
Loaded symbols for /lib/libonion.so
Reading symbols from /lib/libpthread.so.0...done.
Loaded symbols for /lib/libpthread.so.0
Reading symbols from /usr/lib/libstdc++.so.6...done.
Loaded symbols for /usr/lib/libstdc++.so.6
Reading symbols from /lib/libm.so.6...done.
Loaded symbols for /lib/libm.so.6
Reading symbols from /lib/libgcc_s.so.1...done.
Loaded symbols for /lib/libgcc_s.so.1
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/libdl.so.2...done.
Loaded symbols for /lib/libdl.so.2
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
Core was generated by `./a.out‘.
Program terminated with signal 11, Segmentation fault.
#0 0x08048cc0 in __gnu_cxx::new_allocator<int>::construct (this=0x804a200, __p=0xb6cc2000, [email protected])
at /usr/include/c++/4.1.2/ext/new_allocator.h:104
104 { ::new(__p) _Tp(__val); }
(gdb) bt
#0 0x08048cc0 in __gnu_cxx::new_allocator<int>::construct (this=0x804a200, __p=0xb6cc2000, [email protected])
at /usr/include/c++/4.1.2/ext/new_allocator.h:104
#1 0x08049846 in std::vector<int, std::allocator<int> >::push_back (this=0x804a200, [email protected])
at /usr/include/c++/4.1.2/bits/stl_vector.h:606
#2 0x08048bde in fun (p=0x0) at test.cpp:16
#3 0xb7f471eb in start_thread () from /lib/libpthread.so.0
#4 0xb7da97fe in clone () from /lib/libc.so.6
(gdb) f 2
#2 0x08048bde in fun (p=0x0) at test.cpp:16
16 g_v.push_back(i);
(gdb) i locals
i = 63854
(gdb) i args
p = (void *) 0x0
(gdb) f 1
#1 0x08049846 in std::vector<int, std::allocator<int> >::push_back (this=0x804a200, [email protected])
at /usr/include/c++/4.1.2/bits/stl_vector.h:606
606 this->_M_impl.construct(this->_M_impl._M_finish, __x);
(gdb) i locals
No locals.
(gdb) i args
this = (std::vector<int,std::allocator<int> > * const) 0x804a200
__x = (const int &) @0xb7ce2464: 63854
(gdb) p this
$1 = (std::vector<int,std::allocator<int> > * const) 0x804a200
(gdb) p *this
$2 = {<std::_Vector_base<int,std::allocator<int> >> = {
_M_impl = {<std::allocator<int>> = {<__gnu_cxx::new_allocator<int>> = {<No data fields>}, <No data fields>}, _M_start = 0xb6c81008,
_M_finish = 0xb6cc2000, _M_end_of_storage = 0xb6cc1008}}, <No data fields>}
(gdb)
重点关注frame 1, 其中有:_M_start, _M_finish, _M_end_of_storage, 熟悉vector底层动态分配的朋友, 应该能猜出这三个变量的含义, _M_start指向vector头, _M_finish指向vector尾, _M_end_of_storage指向预分配内存的尾。 来看下vector的push_back函数源码:
void
push_back(const value_type& __x)
{
if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
{
_Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish, __x);
++this->_M_impl._M_finish;
}
else
#if __cplusplus >= 201103L
_M_emplace_back_aux(__x);
#else
_M_insert_aux(end(), __x);
#endif
}
可以看到, 在单线程环境下, 执行push_back的时候, _M_finish总是逐渐去追逐最后的_M_end_of_storage,,容量不够时继续扩_M_end_of_storage, 总之,_M_finish不会越过_M_end_of_storage. 但是, 在多线程环境下, 当_M_finish比_M_end_of_storage小1时,可能会出现多线程同时满足this->_M_impl._M_finish != this->_M_impl._M_end_of_storage, 然后同时执行++this->_M_impl._M_finish, 这样,_M_finish就越过了_M_end_of_storage, 如我们实验中的例子那样。越界操作导致有coredump。 当然, 具体是否越过, 是概率性的, 我们要避免这种未定义行为。
怎么办呢? 可以考虑加锁, 把上述程序的注释取消, 也就是加了互斥锁(mutex), 实际多次运行发现, 再也没有coredump了。
还有一个问题: 上面的结论是_M_finish越过了_M_end_of_storage, 导致coredump, 那如果让_M_end_of_storage不被越过呢? 理论上认为,不会core dump, 如下:
#include <pthread.h>
#include <unistd.h>
#include <iostream>
#include <vector>
#define N 2
using namespace std;
vector<int> g_v;
pthread_mutex_t mutex;
void* fun(void *p)
{
for(int i = 0; i < 100000; i++)
{
//pthread_mutex_lock(&mutex);
g_v.push_back(i);
//pthread_mutex_unlock(&mutex);
}
return NULL;
}
int main()
{
g_v.reserve(999999); // pay attention
pthread_t threads[ N];
pthread_mutex_init(&mutex, NULL);
for(int i = 0; i < N; i++)
{
pthread_create(&threads[i], NULL, fun, NULL);
}
for(int i = 0; i < N; i++)
{
pthread_join(threads[i],NULL);
}
cout << "ok" << endl;
return 0;
}
编译并运行多次, 未见coredump. 尽管如此, 也不能完全保证上述操作的结果符合预期的逻辑, 毕竟,多线程还在操作着非原子的push_back呢。
最后,回到我遇到的那个问题,定时更新配置,可以考虑加锁。如果不用锁, 该怎么实现呢? 可以考虑用两个vector, 轮换使用,更新的vector不去读, 当前的读的vector不更新,然后轮换当前vector. 我见过很多地方都是这么用的。
类似的问题还有很多很多, 坑, 就在那里, 不多不少。 书本Effective STL第12 条如是说:切勿对STL 容器的线程安全性有不切实际的依赖!
双缓冲队列文章,注意双缓冲队列不是完全不加锁 而是尽可能的减少加减锁
https://www.cnblogs.com/Forever-Kenlen-Ja/p/7811943.html
http://www.cnblogs.com/cqgis/p/6403262.html
原文地址:https://www.cnblogs.com/wangshaowei/p/10715355.html