There are two race conditions of subDirs in DiskBlockManager:
1. `getAllFiles` does not use correct locks to read the contents in `subDirs`. Although it's designed for testing, it's still worth to add correct locks to eliminate the race condition.
2. The double-check has a race condition in `getFile(filename: String)`. If a thread finds `subDirs(dirId)(subDirId)` is not null out of the `synchronized` block, it may not be able to see the correct content of the File instance pointed by `subDirs(dirId)(subDirId)` according to the Java memory model (there is no volatile variable here).