[PATCH tip/core/rcu 0/3] rcu: simplify rcu_barrier() interaction with CPU hotplug

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

This patchset simplifies the interaction of rcu_barrier() with CPU
hotplug operations.  The main point of this patchset is to impose an
invariant: offline CPUs never have any RCU callbacks queued.  However,
rcutiny doesn't permit CPU hotplug (yes, I have worked in environments
where the last CPU could be offlined, but Linux is thankfully not one of
them), so the patchset goes as follows:

o	Replace the rcu_barrier enum with a pointer to the relevant
	call_rcu() function, thus eliminating any confusion about
	which .h file this enum should reside in.

o	Move the rcu_barrier() code to rcutree, and create a lightweight
	variant of rcu_barrier() that is suitable for rcutiny.  This
	lightweight variant is identical to rcutree's implementation of
	synchronize_rcu() and friends.

o	Create a list in the rcu_state structure that holds RCU
	callbacks that were "orphaned" by CPUs that just went offline.
	The CPU_DYING notifier moves all RCU callbacks from the outgoing
	CPU to the rcu_state lists, and the CPU_DEAD notifier and
	_rcu_barrier() function "adopt" these orphans.

 b/kernel/rcupdate.c       |   33 +++-------
 b/kernel/rcutiny.c        |   36 ++++++++++
 b/kernel/rcutree.c        |  120 ++++++++++++++++++++++++++++++++++++
 b/kernel/rcutree.h        |   11 +++
 b/kernel/rcutree_plugin.h |   34 ++++++++++
 b/kernel/rcutree_trace.c  |    5 -
 kernel/rcupdate.c         |  120 ------------------------------------
 kernel/rcutree.c          |  151 ++++++++++++++++++++++++----------------------
 8 files changed, 291 insertions(+), 219 deletions(-)