atomic added

Author: behnamfly

docs/atomic.md

@@ -0,0 +1,80 @@
+
+
+The followings will print `200`, since each thread will increase the counter 100,
+```cpp
+int main() {
+  int counter = 0;
+
+  auto work = [&]() {
+    for (int i = 0; i < 100; i++)
+      counter += 1;
+  };
+
+  std::thread t1(work);
+  std::thread t2(work);
+
+  t1.join();
+  t2.join();
+
+  std::cout << counter << std::endl;
+  return 0;
+}
+```
+
+In the above example since the value `100` is very small, both of thread can finish the loop before have any effect on each other (without any interleave), 
+however if we increase the length of the loop, threads might have impact each other, and update the value of counter while the other one has just read it, 
+so we have data race problem.
+
+```cpp
+int main() {
+  int counter = 0;
+
+  auto work = [&]() {
+    for (int i = 0; i < 1000000; i++)
+      counter += 1;
+  };
+
+  std::thread t1(work);
+  std::thread t2(work);
+
+  t1.join();
+  t2.join();
+
+  std::cout << counter << std::endl;
+  return 0;
+}
+```
+which in different run gives us:
+
+```
+1098330
+1261226
+1617594
+1188436
+```
+
+
+The solution is to make the `counter` of type `std::atomic<int>;`
+
+
+```cpp
+int main() {
+  std::atomic<int> counter = 0;
+
+  auto work = [&]() {
+    for (int i = 0; i < 1000000; i++)
+      counter += 1;
+  };
+
+  std::thread t1(work);
+  std::thread t2(work);
+
+  t1.join();
+  t2.join();
+
+  std::cout << counter << std::endl;
+  return 0;
+}
+```
+
+Refs: [1](https://www.youtube.com/watch?v=ZWjVa1kfE7c)