book: revision of ch07 & finish atomics

Update #2

code/7/7.6.bad.example.cpp

@@ -0,0 +1,23 @@
+#include <thread>
+#include <iostream>
+
+int main() {
+    int a = 0;
+    volatile int flag = 0;
+
+    std::thread t1([&]() {
+        while (flag != 1);
+
+        int b = a;
+        std::cout << "b = " << b << std::endl;
+    });
+
+    std::thread t2([&]() {
+        a = 5;
+        flag = 1;
+    });
+
+    t1.join();
+    t2.join();
+    return 0;
+}