Here we are explicitly locking the lower level node with X. Lets say the nodes are like A --> B --> C Step 1: We check if A is locked with anything. Not locked. So we lock it with SIX Step 2: We check if B is locked with anything. Not locked. So we lock it with S Step 3: We check if C is locked with anything. Not locked. So we lock it with X. It does not matter if B has a shared lock before C is locked with X. When we checked it, it wasn't locked. So we proceed.
