Double checked locking.

[Tom Kaitchuck]

I was running a code analyzer and it flagged
org.apache.activemq.kaha.impl.getVmLockSet() as doing unsafe locking.
I'm looking at the code and it's not clear to me what it needs to
protect against. It looks to me like this code is trying to do double
checked locking, which in general is not safe in java. Could someone
with a solid understanding of this code take a look at it?