c++ - 这个C++指针使用线程安全吗？

Question

我是否需要在“p = tmp”之前插入栅栏以避免内存重新排序？由于从线程2的角度来看内存重新排序而不使用fence/atomic/mutex，是否有可能在“(*tmp)[1]=2”之前执行“p=tmp”？

线程 1

extern const std::map<int, int>* p;
auto tmp = new std::map<int, int>;
(*tmp)[1] = 2;
...
(*tmp)[2] = 3;
// do I need insert fence here to make sure above operation on tmp ready before tmp assigned to p?
p = tmp;

线程 2

extern const std::map<int, int>* p; // suppose p initalized with {{1:2}}
assert(p->find(1)->second == 2);

Answer 1

Is it possible "p=tmp" executed before "(*tmp)[1]=2" due to memory reordering from the view of thread 2 without using fence/atomic/mutex?

这会发生吗

应该将 volatile 添加到防止重新排序的列表中，但 volatile 仍然无法防止数据争用

Do I need insert fence before "p = tmp" to avoid memory reordering?

您需要添加同步，但栅栏通常不是最佳的，并且是否需要它们是特定于体系结构的。原子更适合这种情况

与#include <atomic>

线程 1

extern std::atomic<const std::map<int, int>*> p;
auto tmp = new std::map<int, int>;
(*tmp)[1] = 2;
...
(*tmp)[2] = 3;
// do I need insert fence here to make sure above operation on tmp ready before tmp assigned to p?
p = tmp;

线程 2

extern std::atomic<const std::map<int, int>*> p; // suppose p initalized with {{1:2}}
assert(p->find(1)->second == 2);