[PATCH tip/core/rcu 0/3] rcu: prevent hangs by simplifying rcu_barrier/CPU-hotplug, fix lockdep complaint

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

This patchset fixes a couple of issues with TREE_PREEMPT_RCU:

1.	The current implementation of rcu_barrier() waits for any
	prior CPU-hotplug operation to complete.  This code path has
	resulted in some hangs, so first move this code from rcupdate.c
	to rcutree.c and then use a simpler algorithm that avoids the
	waiting, eliminating the possibility of such hangs.

2.	The lockdep facility complains from time to time due to the
	fact that certain combinations of CPU-hotplug operations can
	cause the TREE_PREEMPT_RCU code to acquire the root rcu_node
	structure's lock while holding a leaf rcu_node structure's
	lock.  This patch places the root rcu_node structure's lock
	into its own lockdep class to prevent this false positive.

Ingo, please rewind tip/core/rcu to commit #135c8aea before applying this
series, as these patches really need to go into 2.6.32.  Reworked and
retested rcutiny (for 2.6.33, as Linus requested) will follow in a
few days.  Or more patches for more bug fixes, as the case may be.  ;-)

 b/kernel/rcupdate.c       |  120 -----------------------------------
 b/kernel/rcutree.c        |  120 +++++++++++++++++++++++++++++++++++
 b/kernel/rcutree.h        |   11 ++-
 b/kernel/rcutree_plugin.h |   34 ++++++++++
 b/kernel/rcutree_trace.c  |    5 -
 kernel/rcutree.c          |  156 ++++++++++++++++++++++++----------------------
 6 files changed, 248 insertions(+), 198 deletions(-)