sighand's atomic count, to piggyback on its cacheline), and two functions
in kernel/cgroup.c (for now) for easily+safely obtaining and releasing it.

If another part of the kernel later wants to use such a locking mechanism,
the CONFIG_CGROUPS ifdefs should be changed to a higher-up flag that
CGROUPS and the other system would both depend on, and the lock/unlock
functions could be moved to sched.c or so.
Signed-off-by: Ben Blum <bblum@google.com>
Signed-off-by: Paul Menage <menage@google.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>