Pull cgroup updates from Tejun Heo:

 - threadgroup_lock got reorganized so that its users can pick the
   actual locking mechanism to use.  Its only user - cgroups - is
   updated to use a percpu_rwsem instead of per-process rwsem.

   This makes things a bit lighter on hot paths and allows cgroups to
   perform and fail multi-task (a process) migrations atomically.
   Multi-task migrations are used in several places including the
   unified hierarchy.

 - Delegation rule and documentation added to unified hierarchy.  This
   will likely be the last interface update from the cgroup core side
   for unified hierarchy before lifting the devel mask.

 - Some groundwork for the pids controller which is scheduled to be
   merged in the coming devel cycle.

* 'for-4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cgroup: add delegation section to unified hierarchy documentation
  cgroup: require write perm on common ancestor when moving processes on the default hierarchy
  cgroup: separate out cgroup_procs_write_permission() from __cgroup_procs_write()
  kernfs: make kernfs_get_inode() public
  MAINTAINERS: add a cgroup core co-maintainer
  cgroup: fix uninitialised iterator in for_each_subsys_which
  cgroup: replace explicit ss_mask checking with for_each_subsys_which
  cgroup: use bitmask to filter for_each_subsys
  cgroup: add seq_file forward declaration for struct cftype
  cgroup: simplify threadgroup locking
  sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem
  sched, cgroup: reorganize threadgroup locking
  cgroup: switch to unsigned long for bitmasks
  cgroup: reorganize include/linux/cgroup.h
  cgroup: separate out include/linux/cgroup-defs.h
  cgroup: fix some comment typos