Hung Vacuum in 8.3

This is 8.3.14 on Debian Lenny x86-64.

I'm seeing a hung vacuum:

postgres=# select procpid, query_start,waiting, current_query from
pg_stat_activity where current_query like '%VACUUM%';
procpid | query_start | waiting
|
current_query
--------+-------------------------------+---------+---------------------------------------------------------------------------------------------
7347 | 2011-02-22 06:02:02.400247+13 | f | VACUUM ANALYZE;

at approx 1300 (so it has been sitting there for approx 7 hours, normal
database vacuum time is 10 minutes). Now according to pg_stat_activity
and pg_locks it is *not* waiting for a lock, but no vacuuming appears to
be going on. strace says:

$ strace -p 7347

Process 7347 attached - interrupt to quit
semop(33456157, 0x7ffff512bad0, 1

Ok, so we are waiting on a semaphore - hmm, why is it not showing up as
waiting on a lock of some kind?

Of interest is this:

postgres=# select procpid, query_start, current_query from
pg_stat_activity order by query_start limit 1;
procpid | query_start
| current_query
---------+-------------------------------+-------------------------------------------------------------------------------------
25953 | 2011-02-22 04:24:07.417138+13 | SELECT n.node_id, n.node_ --
long query, joining several large tables - text snipped

So this guy had been running from before the vacuum started, so probably
vacuum is wanting to do lazy_truncate_heap() on one of the tables in the
join (no gdb on this box unfortunately). I am however still puzzled
about why no locks are being waited on.

I have canceled the vacuum, but any suggestions for getting more diag
info for next time?

regards

Mark