The problem is inside the lock-free queue. How do you remove something from a lock-free queue? Usually by reading a pointer to the queue node, then doing a CA

The problem is inside the lock-free queue. How do you remove something from a lock-free queue? Usually by reading a pointer to the queue node, then doing a CAS to remove the node from the queue. But another thread can remove that same node and free it before you can do the CAS! If you're just using a lock-free queue, then the hardest part of the problem has been solved for you already. But the person who wrote the lock-free queue had to think about and solve these really tricky problems.