Add lockless queue type

Single producer, single consumer.

include/osv/lockless-queue.hh

+#ifndef LOCKLESS_QUEUE_HH_
+#define LOCKLESS_QUEUE_HH_
+
+#include <atomic>
+
+template <class T> class lockless_queue_link;
+template <class T, lockless_queue_link<T> T::*link> class lockless_queue;
+
+// single producer, single consumer, lockless
+
+template <class T, lockless_queue_link<T> T::*link>
+class lockless_queue {
+public:
+    lockless_queue();
+    bool empty() const;
+    void copy_and_clear(lockless_queue& to);
+    void push_front(T& elem);
+    T& front();
+    void pop_front_nonatomic();
+private:
+    std::atomic<T*> _head;
+};
+
+template <class T>
+class lockless_queue_link
+{
+public:
+    T* _next;
+};
+
+template <class T, lockless_queue_link<T> T::*link>
+lockless_queue<T, link>::lockless_queue()
+    : _head(nullptr)
+{
+
+}
+
+template <class T, lockless_queue_link<T> T::*link>
+void lockless_queue<T, link>::copy_and_clear(lockless_queue& to)
+{
+    to._head = _head.exchange(nullptr);
+}
+
+template <class T, lockless_queue_link<T> T::*link>
+void lockless_queue<T, link>::push_front(T& elem)
+{
+    // no ABA, since we are sole producer
+    T* old;
+    do {
+        old = _head.load();
+        (elem.*link)._next = old;
+    } while (!_head.compare_exchange_weak(old, &elem));
+}
+
+template <class T, lockless_queue_link<T> T::*link>
+bool lockless_queue<T, link>::empty() const
+{
+    return !_head.load();
+}
+
+template <class T, lockless_queue_link<T> T::*link>
+T& lockless_queue<T, link>::front()
+{
+    return *_head.load();
+}
+
+template <class T, lockless_queue_link<T> T::*link>
+void lockless_queue<T, link>::pop_front_nonatomic()
+{
+    _head = (_head.load()->*link)._next;
+}
+
+#endif /* LOCKLESS_QUEUE_HH_ */