profile_hook unconditionally takes a read lock on profile_lock if kernel
profiling is enabled.  The lock protects the profile_hook notifier chain
from being written while it's being called.  The routine profile_hook is
called in a very hot path though: every timer tick on every CPU.  As you
can imagine, on a large system, this makes the cacheline containing
profile_lock pretty hot.  Since oprofile was the only user of the
profile_hook, I removed the notifier chain altogether in favor of a simple
function pointer with the help of John Levon.  This removes all of the
contention in the hot path since the variable is very seldom written and
simplifies things a little to boot.
Acked-by: John Levon <levon@movementarian.org>
Signed-off-by: Jesse Barnes <jbarnes@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>