memcg_offline_kmem() may be called from memcg_free_kmem() after a css
init failure.  memcg_free_kmem() is a ->css_free callback which is
called without cgroup_mutex and memcg_offline_kmem() ends up using
css_for_each_descendant_pre() without any locking.  Fix it by adding rcu
read locking around it.

    mkdir: cannot create directory `65530': No space left on device
    ===============================
    [ INFO: suspicious RCU usage. ]
    4.6.0-work+ #321 Not tainted
    -------------------------------
    kernel/cgroup.c:4008 cgroup_mutex or RCU read lock required!
     [  527.243970] other info that might help us debug this:
     [  527.244715]
    rcu_scheduler_active = 1, debug_locks = 0
    2 locks held by kworker/0:5/1664:
     #0:  ("cgroup_destroy"){.+.+..}, at: [<ffffffff81060ab5>] process_one_work+0x165/0x4a0
     #1:  ((&css->destroy_work)#3){+.+...}, at: [<ffffffff81060ab5>] process_one_work+0x165/0x4a0
     [  527.248098] stack backtrace:
    CPU: 0 PID: 1664 Comm: kworker/0:5 Not tainted 4.6.0-work+ #321
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.1-1.fc24 04/01/2014
    Workqueue: cgroup_destroy css_free_work_fn
    Call Trace:
      dump_stack+0x68/0xa1
      lockdep_rcu_suspicious+0xd7/0x110
      css_next_descendant_pre+0x7d/0xb0
      memcg_offline_kmem.part.44+0x4a/0xc0
      mem_cgroup_css_free+0x1ec/0x200
      css_free_work_fn+0x49/0x5e0
      process_one_work+0x1c5/0x4a0
      worker_thread+0x49/0x490
      kthread+0xea/0x100
      ret_from_fork+0x1f/0x40

Link: http://lkml.kernel.org/r/20160526203018.GG23194@mtj.duckdns.orgSigned-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: <stable@vger.kernel.org>	[4.5+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>