deadlocks - sharelocks on transactions

Hi,

I have a stored procedure that is causing deadlocks when called multiple
times synchronously. The odd issue is that the deadlock seems to be
happening on different threads waiting for locks on transactions. What
exactly is this transaction lock?

ERROR:  deadlock detected
DETAIL:  Process 1740 waits for ShareLock on transaction 1488; blocked
by process 1716.
         Process 1716 waits for ShareLock on transaction 1490; blocked
by process 1740.

It's a little difficult to debug this issue when it's not identifying
which data accesses are causing the deadlocks.

Does anyone have any information that may help in tracking down the problem?

Many thanks,

Tim

Current set-up:

Postgresql 7.4.1 (Windows 2000, Cygwin) or
Postgresql 7.4   (Linux, Redhat 9)