Make RCU synchronous grace periods work throughout boot

Hello!

This series fixes a regression in v4.9 where code relied on RCU synchronous grace periods working between the time that the scheduler spawned the first task and RCU finished spawning all of its tasks. This series makes RCU able to handle synchronous grace periods throughout boot. Please see the second patch's commit log for more details.

The patches are as follows:

1. Remove cond_resched() from Tiny synchronize_sched(), thus allowing
Tiny RCU's synchronize_sched() to be used throughout boot without annoying splats.

2. Make RCU's expedited synchronous grace periods work throughout
boot, and promote non-expedited grace periods to expedited grace periods during boot from the time that the scheduler spawns the first task until after RCU has spawned all of its kthreads.

Changes from v1:

o Added the fix for Tiny RCU.

o Applied Borislav Petkov's feedback for Tree RCU.

o Added a bunch of Tested-by lines.

Thanx, Paul

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

 include/linux/rcupdate.h |    4 +++
 kernel/rcu/rcu.h         |    1 
 kernel/rcu/tiny.c        |    4 ---
 kernel/rcu/tiny_plugin.h |    9 ++++++--
 kernel/rcu/tree.c        |   33 ++++++++++++++++++-----------
 kernel/rcu/tree_exp.h    |   52 +++++++++++++++++++++++++++++++++++++----------
 kernel/rcu/tree_plugin.h |    2 -
 kernel/rcu/update.c      |   38 +++++++++++++++++++++++++++-------
 8 files changed, 104 insertions(+), 39 deletions(-)