This is the RCU core patch from akpm's tree. It has been in his
tree since about 2.5.37-mm1 along with dcache_rcu and so far it has
worked fine. For 2.5, I am hoping that we might get the following
RCU patches included -

1. rt_rcu - ipv4 routecache lookup. Davem agreed to include this patch
   if and when you include RCU core in your tree.

2. dcache_rcu (by Maneesh Soni) - dcache lookup avoiding dcache_lock as
   much as possible. This has been akpm's tree - stable and gives us
   good yield. I have been submitting this to Viro and I will publish
   some more benchmark numbers later to help decide on this.

This RCU core implements RCU APIs, call_rcu() and synchronize_kernel(),
by monitoring a per-CPU quiescent state (idle/user etc.) counter.
call_rcu() queues a callback to be invoked after all the CPUs have
gone through a quiescent state. Queuing is per-CPU and each per-CPU
batch gets a batch number. As batches get their turn, a global
cpu mask is used to keep track of CPUs pending quiescent state.
Checking for quiescent cycle is done by saving the per-CPU
counter at the beginning of the batch and then monitoring it for change
through the local timer interrupt handler.