RMAN blocked by ARCH process, waiting “enq: WL - contention”

Today, we got a file system alert, one server's arch mount point is 90% full.

After checking found one DB's archivelog occupied 90% space.
And this DB's backup arch job was running since 10 days ago and still hang there, no progress.

After checking in the DB, found the rman job was blocked by below red part event:

      SSID    OSUSER     USERNAME          STATE      STATUS                    MODULE
---------- --------- ------------ -------------- ------------ -------------------------                                                  

BLOCKING_SESSION                   EVENT                          PROGRAM
---------------- ----------------------- --------------------------------
       244    oracle          SYS        WAITING       ACTIVE  rman@server621 (TNS V1-V3)
              58     enq: WL - contention       rman@server621 (TNS V1-V3)
SQL_TEXT
--------------------------------------
begin dbms_rcvman.setDatabase(upper(:dbname), :rlscn, :rltime, :fhdbi, :db_unique_name, TRUE); dbms_rcvman.setCanApplyAnyRedo(TRUE); dbms_rcvman.setCanConvertCf(TRUE); if (:canhandletts = 1) then   dbms_rcvman.setCanHandleTransportableTbs(TRUE); end if; end;

It is blocked by session 58, which is ARC0 process:

SID     SPID                        ROGRAM
------ -------- -------------------------- 
    58    10899    oracle@server621 (ARC1)

Killing ARC0 process, oracle will restart it automaticlly.
Then the rman backup job began to progress normally.