locking, lock free and wait free

locking --- lock_guard + mutex
lock free --- atomic + CAS
wait free --- atomic +fetch_add
ring buffer-- producer makes progress without any wait, so wait-free.


using namespace std;
typedef unsigned long ulong;

ulong _shared = 123UL;
int main() {
 {
  ulong* p=nullptr;  //locking
  *p = _shared;
  mutex M;
  {
   lock_guard<mutex> l(M);
   ++(*p);
  }
 }
 atomic<ulong>* p=0;  //lock free
 *p = _shared;
 ulong xl = p->load(memory_order_relaxed);
 while (!p->compare_exchange_strong(xl, xl + 1, std::memory_order_relaxed)) {}

 p->fetch_add(1, memory_order_relaxed);  //wait free


 // cache line is 64 bytes, ul is 8 bytes, so thread>8=> sit on separate cache line, no sharing faster.

 

 return 0;
}