From: Manfred Spraul <manfred@colorfullife.com>

fasync_helper and kill_fasync are helpers for managing F_SETFL fcntl 
calls that change FASYNC and sending the necessary signals. The locking 
uses one global rwlock that's acquired for read in all kill_fasync
calls, and that causes cache line trashing. This is not necessary: if
the fasync list is empty, then there is no need to acquire the rwlock.
Tests with reaim on a 4-way pIII on STP showed an 80% reduction of the
time within kill_fasync.