Enforce RCU grace-period transitivity

Hello!

Within RCU, acquisitions of the rcu_node structure's ->lock field must be followed by smp_mb__after_unlock_lock() in order to enforce RCU's grace-period memory-ordering guarantees. In theory, acquisitions for debug purposes need not have smp_mb__after_unlock_lock(), but in practice, anything other than fully consistent enforcement of this rule is a recipe for disaster. This series therefore adds smp_mb__after_unlock_lock() for consistency:

1. Create wrapper functions to minimize the number of these
smp_mb__after_unlock_lock() calls, and apply these wrapper functions throughout. Courtesy of Peter Zijlstra.

2. Catch up wrapper-function application to patches that were
in flight at the time #1 was created.

Thanx, Paul

------------------------------------------------------------------------

 b/kernel/rcu/tree.c        |  102 ++++++++++++++++-----------------------------
 b/kernel/rcu/tree.h        |   39 +++++++++++++++++
 b/kernel/rcu/tree_plugin.h |   20 +++-----
 b/kernel/rcu/tree_trace.c  |    2 
 4 files changed, 85 insertions(+), 78 deletions(-)