Are Enqueue locks implemented with a FIFO queuing mechanism

Oracle 11gR1 EE on RHEL 4 (x86_64)

Are Oracle enqueue locks obtained in a 'first in, first out' fashion..? I'm specifically interested in 'enqueue - JI' locks - i.e. the exclusive lock obtained to serialzie a materialzized view refresh.

We had a situation recently where a session was waiting to obtain an 'enqeue - JI' resource and was being blocked by sessions which were established after the first session requested access to the lock in question. This behaviour seems unusual (buggy..?) because I expected a fifo type implementation with enqueue resources in Oracle.                                                  

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