how to "refresh" the in-core partition table?

[Robert P. J. Day]

  historically, after i create a new partition, i typically have to
reboot to refresh the kernel's understanding of the partition table.
under what circumstances is this actually necessary?  i know that it's
not *always* necessary.  is it just for primary partitions?  and is
there a way to "force" a refresh on a running system so that rebooting
isn't required?  thanks.

rday