[cgroups] suspicious rcu_dereference_check() usage!

[Mike Galbraith]

Running LTP on master.today (v4.10) with a seriously bloated PREEMPT
config inspired box to emit the below.

[ 7160.458996] ===============================
[ 7160.463195] [ INFO: suspicious RCU usage. ]
[ 7160.467387] 4.10.0-default #100 Tainted: G            E  
[ 7160.472808] -------------------------------
[ 7160.476999] ./include/linux/cgroup.h:435 suspicious rcu_dereference_check() usage!
[ 7160.484576] 
[ 7160.484576] other info that might help us debug this:
[ 7160.484576] 
[ 7160.492577] 
[ 7160.492577] rcu_scheduler_active = 2, debug_locks = 1
[ 7160.499113] 1 lock held by pids_task1/19308:
[ 7160.503390]  #0:  (&cgroup_threadgroup_rwsem){+++++.}, at: [<ffffffff81095b60>] _do_fork+0xf0/0x710
[ 7160.512450] 
[ 7160.512450] stack backtrace:
[ 7160.516810] CPU: 5 PID: 19308 Comm: pids_task1 Tainted: G            E   4.10.0-default #100
[ 7160.525239] Hardware name: IBM System x3550 M3 -[7944K3G]-/69Y5698     , BIOS -[D6E150AUS-1.10]- 12/15/2010
[ 7160.534965] Call Trace:
[ 7160.537414]  dump_stack+0x85/0xc9
[ 7160.540732]  lockdep_rcu_suspicious+0xd5/0x110
[ 7160.545177]  task_css.constprop.7+0x88/0x90
[ 7160.549357]  pids_can_fork+0x132/0x160
[ 7160.553106]  cgroup_can_fork+0x63/0xc0
[ 7160.556855]  copy_process.part.30+0x17ef/0x21b0
[ 7160.561382]  ? _do_fork+0xf0/0x710
[ 7160.564786]  ? free_pages_and_swap_cache+0x9e/0xc0
[ 7160.569575]  _do_fork+0xf0/0x710
[ 7160.572806]  ? __this_cpu_preempt_check+0x13/0x20
[ 7160.577505]  ? __percpu_counter_add+0x86/0xb0
[ 7160.581860]  ? entry_SYSCALL_64_fastpath+0x5/0xc2
[ 7160.586562]  ? do_syscall_64+0x2d/0x200
[ 7160.590395]  SyS_clone+0x19/0x20
[ 7160.593623]  do_syscall_64+0x6c/0x200
[ 7160.597283]  entry_SYSCALL64_slow_path+0x25/0x25
[ 7160.601899] RIP: 0033:0x7fdaa3b881c4
[ 7160.605473] RSP: 002b:00007ffd21635d50 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
[ 7160.613036] RAX: ffffffffffffffda RBX: 0000000000004b6c RCX: 00007fdaa3b881c4
[ 7160.620162] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
[ 7160.627288] RBP: 00007ffd21635d90 R08: 0000000000000000 R09: 00007fdaa4052700
[ 7160.634414] R10: 00007fdaa40529d0 R11: 0000000000000246 R12: 00007ffd21635d50
[ 7160.641539] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000