skiplist synchronization question

The comment in the skiplist.h says like following:
```
void SkipList::Insert(const Key& key) {
// TODO(opt): We can use a barrier-free variant of FindGreaterOrEqual()
// here since Insert() is externally synchronized.
```
But when I following the source code, I found no external synchronization in db_impl.cc:DBImpl::Write.
```
// extract sequence and kType to insert memtable
status = WriteBatchInternal::InsertInto(updates, mem_);
```
Is this comment out of date or I missed something?