CPU hotplug updates for v4.1

Hello!

This series updates RCU's handling of CPU hotplug offline operations, allowing RCU to have a precise notification of when it should start ignoring a CPU. This allowed detection of some illegal use of RCU by offline CPUs, and this series contains fixes for these. A similar problem exists for CPU onlining, but will be addressed later. One CPU-hotplug dragon at a time.

1. Add common code for notification from dying CPU. This is
part of the fix for issues uncovered by improved detection, but is placed first to avoid messing up bisection.

2-4. Use #1 for x86, blackfin, and metag. (ARM also has this problem,
but ARM's maintainers are working on their own fix.)

5. Remove duplicate offline-CPU callback-list initialization.
This simplifies later changes to RCU's handling of offlining operations.

6. Improve code readability in rcu_cleanup_dead_cpu(). Simple
code motion, no semantic change.

7. Eliminate a boolean variable and "if" statement by rearranging
sync_rcu_preempt_exp_init()'s checks for CPUs not having blocked tasks.

8. Eliminate empty CONFIG_HOTPLUG_CPU #ifdef.

9. Add diagnostics to detect when RCU CPU stall warnings have been
caused by failure to propagate quiescent states up the rcu_node combining tree.

10. Provide CONFIG_RCU_TORTURE_TEST_SLOW_INIT Kconfig option to
artificially slow down grace-period initialization, thus increasing the probability of detecting races with this initialization process.

11. Update data files to enable CONFIG_RCU_TORTURE_TEST_SLOW_INIT
by default during rcutorture testing, but leave the default time at zero. This default may be overridden by passing "--bootargs rcutree.gp_init_delay=1" to kvm.sh.

12. Remove event tracing from rcu_cpu_notify(), which is invoked
by offline CPUs. (Event tracing uses RCU.)

13. Change meaning of ->expmask bitmasks to track blocked tasks
rather than online CPUs.

14. Move rcu_report_unblock_qs_rnp() to common code. This will
make it easier to provide proper locking protection.

15. Avoid races between CPU-hotplug operations and RCU grace-period
initialization by processing CPU-hotplug changes only at the start of each grace period. This works because RCU need not wait on a CPU that came online after the start of the current grace period.

16. Eliminate the no-longer-needed ->onoff_mutex from the rcu_node
structure. This is the only sleeplock acquired during RCU's CPU-hotplug processing, which in turn allows rcu_cpu_notify() to be invoked from the preemption-disabled idle loop.

17. Use a per-CPU variable to make the CPU-offline idle-loop
transition point precise. (RCU's magic one-jiffy grace-period wait for offline CPUs must remain until the analogous online issue is addressed.)

18. Invoke rcu_cpu_notify() with a new CPU_DYING_IDLE op just before
the idle-loop invocation of arch_cpu_idle_dead().

19. Now that CPU-hotplug events are applied only during grace-period
initialization, it is safe to unconditionally enable slow grace-period initialization for rcutorture testing. Note that this delay is applied randomly in order to get a good mix of fast and slow grace-period initialization.

20. Add checks that all quiescent states were received at grace-period
cleanup time.

21. Add a check for the last task on a given RCU-node structure
leaving its RCU read-side critical section between the time that hotplug information is propagated up the tree and the time that the grace period starts.

22. Add checks for grace-period number to all propagations of
quiescent states up the rcu_node combining tree. These are required because a new grace period could start during this propagation due to the resolution of #21 above. (Thanks to Sasha Levin for exposing this bug during the course of his testing.)

Changes since v1:

o Fixed per-CPU state mechine to work correctly for architectures
that online CPUs without needing to check whether or not previous offline operations completed correctly and on time, thanks to James Hogan.

o Fixed Xen's interfacing to the common-code notifications, thanks
to Boris Ostrovsky.

o Added two fixes for handling of quiescent states and grace periods
given the updated handling of CPU hotplug.

Thanx, Paul

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

 b/Documentation/kernel-parameters.txt                     |    6 
 b/arch/blackfin/mach-common/smp.c                         |    6 
 b/arch/metag/kernel/smp.c                                 |    5 
 b/arch/x86/include/asm/cpu.h                              |    2 
 b/arch/x86/include/asm/smp.h                              |    2 
 b/arch/x86/kernel/smpboot.c                               |   39 -
 b/arch/x86/xen/smp.c                                      |   46 -
 b/include/linux/cpu.h                                     |   14 
 b/include/linux/rcupdate.h                                |    2 
 b/kernel/cpu.c                                            |    4 
 b/kernel/rcu/tree.c                                       |  356 ++++++++++----
 b/kernel/rcu/tree.h                                       |   11 
 b/kernel/rcu/tree_plugin.h                                |  169 +++---
 b/kernel/rcu/tree_trace.c                                 |    4 
 b/kernel/sched/idle.c                                     |    9 
 b/kernel/smpboot.c                                        |  156 ++++++
 b/lib/Kconfig.debug                                       |   26 -
 b/tools/testing/selftests/rcutorture/configs/rcu/CFcommon |    1 
 18 files changed, 617 insertions(+), 241 deletions(-)