Pull RCU updates from Boqun Feng:

 - Eliminate deadlocks involving do_exit() and RCU tasks, by Paul:
   Instead of SRCU read side critical sections, now a percpu list is
   used in do_exit() for scaning yet-to-exit tasks

 - Fix a deadlock due to the dependency between workqueue and RCU
   expedited grace period, reported by Anna-Maria Behnsen and Thomas
   Gleixner and fixed by Frederic: Now RCU expedited always uses its own
   kthread worker instead of a workqueue

 - RCU NOCB updates, code cleanups, unnecessary barrier removals and
   minor bug fixes

 - Maintain real-time response in rcu_tasks_postscan() and a minor fix
   for tasks trace quiescence check

 - Misc updates, comments and readibility improvement, boot time
   parameter for lazy RCU and rcutorture improvement

 - Documentation updates

* tag 'rcu.next.v6.9' of git://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux: (34 commits)
  rcu-tasks: Maintain real-time response in rcu_tasks_postscan()
  rcu-tasks: Eliminate deadlocks involving do_exit() and RCU tasks
  rcu-tasks: Maintain lists to eliminate RCU-tasks/do_exit() deadlocks
  rcu-tasks: Initialize data to eliminate RCU-tasks/do_exit() deadlocks
  rcu-tasks: Initialize callback lists at rcu_init() time
  rcu-tasks: Add data to eliminate RCU-tasks/do_exit() deadlocks
  rcu-tasks: Repair RCU Tasks Trace quiescence check
  rcu/sync: remove un-used rcu_sync_enter_start function
  rcutorture: Suppress rtort_pipe_count warnings until after stalls
  srcu: Improve comments about acceleration leak
  rcu: Provide a boot time parameter to control lazy RCU
  rcu: Rename jiffies_till_flush to jiffies_lazy_flush
  doc: Update checklist.rst discussion of callback execution
  doc: Clarify use of slab constructors and SLAB_TYPESAFE_BY_RCU
  context_tracking: Fix kerneldoc headers for __ct_user_{enter,exit}()
  doc: Add EARLY flag to early-parsed kernel boot parameters
  doc: Add CONFIG_RCU_STRICT_GRACE_PERIOD to checklist.rst
  doc: Make checklist.rst note that spinlocks are implied RCU readers
  doc: Make whatisRCU.rst note that spinlocks are RCU readers
  doc: Spinlocks are implied RCU readers
  ...