Merge branch 'for-4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

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 com...