enq: TX - contention, alter tablespace read only

-- hang
RMAN> sql 'alter tablespace t_i read only';

sql statement: alter tablespace t_i read only
-- why
select ash.event
, ash.sql_id
, ash.session_id
, ash.blocking_session
, ash.SAMPLE_TIME
from v$active_session_history ash
where ash.event like '%contention%'
order by ash.sample_time
;