enq: TX - contention

Problem:

ALTER TABLESPACE XXX READ ONLY;

Session moves into a state of waiting with Wait Event "enq: TX - contention".

Description:
This is possible if parallel INSERTs are running and one of these INSERTs triggered an AUTOEXTEND operation and must wait for "Data file init write/read" (Note 619188). In this case, the other INSERTs wait for "enq: TX - contention".

In my particular situation there was an INSERT in parallel. Blocking session was in "db file sequential read" state and was reading system datafile.

Solution:
wait until session dies itself or kill it brutally.