Deadlock with ShareLocks?

I've an interesting error with deadlocks on a quite simple table:

Version: 8.1
Query : update last_modified set dataend=now() where type='list'
DB-Error : ERROR: deadlock detected
DETAIL: Process 10454 waits for ShareLock on transaction 1347632; blocked by
process 15920.
Process 15920 waits for ShareLock on transaction 1347633; blocked by process
10454.

I thought ShareLock is not really blocking, or am I wrong? The bad thing is,
that it's quite difficult to get the info what the other backend was doing at
the same time. Maybe it would be a good idea to log the statements of both
backends (if available). Would make detecting such errors quite easier.

Best Regards,
Mario Weilguni