Deadlocks? What happened to MVCC?

Folks:

I'm getting deadlock errors on one of the operations on my web
application.  It's a function which adds a large number of rows to a
holding table, then updates that set of rows multiple times in order to
present scoring information to the user.

However, the function is deadlocking itself once it's in heavy use.  I
though that MVCC was supposed to supercede deadlocks, unless I
specifically set locks, which I'm not.

Help, anyone?

-Josh Berkus

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