What does the wait type PREEMPTIVE_XE_CALLBACKEXECUTE mean?

Question

I have two instances in the last week where a DROP command for an Extended Events session has taken over a day to complete, sitting with the wait: PREEMPTIVE_XE_CALLBACKEXECUTE.

Can anyone explain what this wait type means?

More Background Info: In the first instance, I ran a T-SQL command to drop the session and it completed a day later. Using sp_whoisactive, I saw that the query was waiting on PREEMPTIVE_XE_CALLBACKEXECUTE.

During this time, Object Explorer's queries to gather metadata were being blocked and recieving lock-timeouts, but there were no other complaints (or so I thought at the time).

I tried to drop another session on Friday, and the same behavior occurred, except it didn't just go away after a day as the with the first event. Instead, I found out this morning that the client application could not connect. It was being blocked by the DROP EVENT SESSION query.

A restart of the SQL Service cleared out the blocking query.

I can't find anything that helps to diagnose what this wait type this is... and why my event sessions won't drop like they should. Can you help?

Server Info: SQL 2008 R2 Enterprise with SP2