Add some commments

src/lockfree/list.rs

@@ -7,6 +7,9 @@ use core::sync::atomic::Ordering::*;
 use crossbeam_epoch::{Atomic, Guard, Owned, Shared};
 
 /// Linked list node.
+// TODO: This node type is very brittle; what if some list creates a node, and uses it to add it to
+// another, separate list? Also see the discussions at <https://github.com/kaist-cp/cs431/issues/957>.
+// The public API surface is way too large.
 #[derive(Debug)]
 pub struct Node<K, V> {
     /// Mark: tag(), Tag: not needed

src/lockfree/queue.rs

@@ -121,6 +121,11 @@ impl<T> Queue<T> {
                     .compare_exchange(tail, next, Release, Relaxed, guard);
             }
 
+            // After the above load & CAS, the thread view ensures that the index of tail is greater
+            // than that of current head. We relase that view to the head with the below CAS,
+            // ensuring that the index of the new head is less than or equal to that of the tail.
+            //
+            // Note: similar reasoning is done in SC memory regarding index of head and tail.
             if self
                 .head
                 .compare_exchange(head, next, Release, Relaxed, guard)