RFC Rework expedited grace periods

Hello!

This RFC series reworks the handling of expedited grace periods, eliminating the previous use of try_stop_cpus(), improving scalability, and adding an RCU CPU stall-warning capability. This series is as follows:

1. Replace try_stop_cpus() with stop_one_cpu(), courtesy of
Peter Zijlstra.

2. Improve counter handling to more efficiently detect that some
other task has already done the required work.

3. Replace synchronize_sched_expedited()'s polling loop with
funnel locking.

4. Replace stop_one_cpu() with stop_one_cpu_nowait() to avoid the
latency hit from #1 above, also courtesy of Peter Zijlstra.

5. Abstract the counter handling.

6. Apply the newly abstracted counter handling to
synchronize_rcu_expedited().

7. Abstract the funnel locking.

8. Fix a stupid type error in synchronize_sched_expedited().

9. Replace synchronize_rcu_expedited()'s polling with the newly
abstracted funnel locking.

10. Apply the abstracted counter handling to _rcu_barrier().

11. Consolidate the last expedited open-coded expedited memory barrier
into the counter handling.

12. Extend the funnel locking to the rcu_data structure in order to
further increase scalability.

13. Add stall warnings to synchronize_sched_expedited().

14. Document the newly added stall warnings.

Thanx, Paul

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

 b/Documentation/RCU/stallwarn.txt |   17 +
 b/include/trace/events/rcu.h      |    1 
 b/kernel/rcu/tree.c               |  581 +++++++++++++++++++-------------------
 b/kernel/rcu/tree.h               |   31 +-
 b/kernel/rcu/tree_plugin.h        |   53 +--
 b/kernel/rcu/tree_trace.c         |   25 -
 6 files changed, 369 insertions(+), 339 deletions(-)