Dead lock

Hi all,
I have 2 data bases trying to perform an update query at the same time on a same table in a third data base using db link.
I'm getting a dead lock exception:
ERROR:  deadlock detected
DETAIL:  Process 27305 waits for ShareLock on transaction 55575; blocked by process 27304.
Process 27304 waits for ShareLock on transaction 55576; blocked by process 27305.
HINT:  See server log for query details.
Actually the folowing function is installed on 2 dbs DB1 and DB2. This function issues an update query on DB3.
When this function is running simultaneously on DB1 and DB2, it produces a dead lock making one of the functions (in DB1 or DB2) stop with the above exception:
Is it normal? should'nt postgres be able to handle such situations, for ex:  let one transaction wait untill the other commits or rollback then continue with the first transaction?
Is there a parameter that should be set in postgresql.conf to allow handling of concurrent transaction...?

CREATE OR REPLACE FUNCTION TEST_DB_LINK(VARCHAR)
RETURNS VOID AS'
DECLARE
C INTEGER;
P ALIAS FOR $1;
DUMMY  VARCHAR;
BEGIN   
    C:= 0;
    LOOP
        EXIT WHEN C > 15;
        C:= C+1;
        SELECT INTO DUMMY DBLINK_EXEC(''CONNECTION_STRING TO DB3', ''UPDATE IN_FICHE_PRODUIT SET VALIDE = 1'');
    RAISE NOTICE ''%, %'', C,P;
    END LOOP;
END;'
LANGUAGE 'plpgsql';

Thanks for your time.