rcu: classic RCU locking and memory-barrier cleanups

rcu: classic RCU locking and memory-barrier cleanups

This patch simplifies the locking and memory-barrier usage in the Classic
RCU grace-period-detection mechanism, incorporating Lai Jiangshan's
feedback from the earlier version (http://lkml.org/lkml/2008/8/1/400
and http://lkml.org/lkml/2008/8/3/43).  Passed 10 hours of
rcutorture concurrent with CPUs being put online and taken offline on
a 128-hardware-thread Power machine.  My apologies to whoever in the
Eastern Hemisphere was planning to use this machine over the Western
Hemisphere night, but it was sitting idle and...

So this is ready for tip/core/rcu.

This patch is in preparation for moving to a hierarchical
algorithm to allow the very large SMP machines -- requested by some
people at OLS, and there seem to have been a few recent patches in the
4096-CPU direction as well.  The general idea is to move to a much more
conservative concurrency design, then apply a hierarchy to reduce
contention on the global lock by a few orders of magnitude (larger
machines would see greater reductions).  The reason for taking a
conservative approach is that this code isn't on any fast path.

Prototype in progress.

This patch is against the linux-tip git tree (tip/core/rcu).  If you
wish to test this against 2.6.26, use the following set of patches:

http://www.rdrop.com/users/paulmck/patches/2.6.26-ljsimp-1.patch
http://www.rdrop.com/users/paulmck/patches/2.6.26-ljsimpfix-3.patch

The first patch combines commits 5127bed588a2f8f3a1f732de2a8a190b7df5dce3
and 3cac97cbb14aed00d83eb33d4613b0fe3aaea863 from Lai Jiangshan
<laijs@cn.fujitsu.com>, and the second patch contains my changes.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>