book: revision of ch07 & finish atomics

Update #2

exercises/7/7.2.mutex.cpp

@@ -0,0 +1,44 @@
+#include <atomic>
+#include <thread>
+#include <iostream>
+
+class mutex {
+    std::atomic<bool> flag{false};
+
+public:
+    void lock()
+    {
+        while (flag.exchange(true, std::memory_order_relaxed));
+        std::atomic_thread_fence(std::memory_order_acquire);
+    }
+
+    void unlock()
+    {
+        std::atomic_thread_fence(std::memory_order_release);
+        flag.store(false, std::memory_order_relaxed);
+    }
+};
+
+int a = 0;
+
+int main() {
+
+    mutex mtx_a;
+
+    std::thread t1([&](){
+        mtx_a.lock();
+        a += 1;
+        mtx_a.unlock();
+    });
+    std::thread t2([&](){
+        mtx_a.lock();
+        a += 2;
+        mtx_a.unlock();
+    });
+
+    t1.join();
+    t2.join();
+
+    std::cout << a << std::endl;
+    return 0;
+}