locking primatives

[Colin Percival]

  I'm trying to implement a callout_stop_safe() function
(same as callout_stop(), except that it would wait for the
callout to finish if it is currently in progress) in order
to get around a rather nasty panic in the kqueue code.  All
I need to do is block until the softclock thread reaches a
specified point -- this is pretty much the simplest locking
problem possible.
  Is there any introduction to the various locking methods
which would give me some indication of which one I should
use?

Colin Percival