why my delete command blocked....

hi, all:
I use a plproxy to execute sql like below:
CREATE OR REPLACE FUNCTION public.dmlexec(query text,op text) RETURNS
setof integer AS $$
CLUSTER 'testcluster';
RUN ON all;
$$ LANGUAGE plproxy;
create or replace function public.dmlExec(query text, op text) returns
integer as $$
declare
ret integer;
begin
execute query;
return 1;
end;
$$ language plpgsql;

when I exectue sql like 'select dmlexec('delete from table_test where
id = 2', 'test') at the same time in numerous process, the delete sql
will blocked.....
so I must select pg_cancel_backend(...).

is there a deadlock??

postgresql version: 8.3.0

thanks all:)