apparent lock contention

Under moderate levels of I/O and light processing load, I
experience what appears to be severe contention on Giant
with several processes at a time holding in state "*Giant*"
for several seconds each.

The problem was initiated by a umount operation on a UFS fs
which completed successfully after about ten minutes of
whirling.

This is a SMP system running 5.2-R.

Can anyone tell me how I might better diagnose what is going
on?

Thanks.