random isolation test failures

We are seeing numerous occasional buildfarm failures of the fk-deadlock2 
isolation test, that look like this:
   ***************   *** 32,39 ****      step s2u1: UPDATE B SET Col2 = 1 WHERE BID = 2;      step s1u2: UPDATE B SET
Col2= 1 WHERE BID = 2;<waiting ...>      step s2u2: UPDATE B SET Col2 = 1 WHERE BID = 2;   - step s1u2:<... completed>
   ERROR:  deadlock detected      step s2c: COMMIT;      step s1c: COMMIT;
 
   --- 32,39 ----      step s2u1: UPDATE B SET Col2 = 1 WHERE BID = 2;      step s1u2: UPDATE B SET Col2 = 1 WHERE BID
=2;<waiting ...>      step s2u2: UPDATE B SET Col2 = 1 WHERE BID = 2;      ERROR:  deadlock detected   + step s1u2:<...
completed>     step s2c: COMMIT;      step s1c: COMMIT;
 


If this is harmless, we could provide an alternative results file as a 
simple fix. If it's not harmless, it should be fixed.

cheers

andrew