Since each DeleteSlice maintains its own head and the list is only single linked the garbage collector takes care of pruning the list for us. All nodes in the list that are still relevant should be either directly or indirectly referenced by one of the DWPT's private DeleteSlice or by the global BufferedUpdates slice.
Each DWPT as well as the global delete pool maintain their private DeleteSlice instance. In the DWPT case updating a slice is equivalent to atomically finishing the document. The slice update guarantees a "happens before" relationship to all other updates in the same indexing session. When a DWPT updates a document it: