cgroup: fix invalid rcu dereference

cgroup: fix invalid rcu dereference

Commit ad676077a2ae ("device_cgroup: convert device_cgroup internally to
policy + exceptions") removed rcu locks which are needed in task_devcgroup
called in this chain: devcgroup_inode_mknod OR
__devcgroup_inode_permission -> __devcgroup_inode_permission ->
task_devcgroup -> task_subsys_state -> task_subsys_state_check.

Change the code so that task_devcgroup is safely called with rcu read
lock held.

===============================
[ INFO: suspicious RCU usage. ]
3.6.0-rc5-next-20120913+ #42 Not tainted
-------------------------------
include/linux/cgroup.h:553 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 0
2 locks held by kdevtmpfs/23:
 #0:  (sb_writers){.+.+.+}, at: [<ffffffff8116873f>]
mnt_want_write+0x1f/0x50
 #1:  (&sb->s_type->i_mutex_key#3/1){+.+.+.}, at: [<ffffffff811558af>]
kern_path_create+0x7f/0x170

stack backtrace:
Pid: 23, comm: kdevtmpfs Not tainted 3.6.0-rc5-next-20120913+ #42
Call Trace:
 [<ffffffff810c638d>] lockdep_rcu_suspicious+0xfd/0x130
 [<ffffffff8121541d>] devcgroup_inode_mknod+0x19d/0x240
 [<ffffffff8107bf54>] ? ns_capable+0x44/0x80
 [<ffffffff81156b21>] vfs_mknod+0x71/0xf0
 [<ffffffff813a8332>] handle_create.isra.2+0x72/0x200
 [<ffffffff813a85d4>] devtmpfsd+0x114/0x140
 [<ffffffff813a84c0>] ? handle_create.isra.2+0x200/0x200
 [<ffffffff81093ad6>] kthread+0xd6/0xe0
 [<ffffffff81654f24>] kernel_thread_helper+0x4/0x10
 [<ffffffff8165369d>] ? retint_restore_args+0xe/0xe
 [<ffffffff81093a00>] ? kthread_create_on_node+0x140/0x140
 [<ffffffff81654f20>] ? gs_change+0xb/0xb

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Dave Jones <davej@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: James Morris <jmorris@namei.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>