Is there a reason why the "online" file in the subdirectories for the CPUs
in /sys/devices/system isn't world-readable?  I cannot imagine it to be
security relevant especially now that a getcpu() syscall can be used to
determine what CPUa thread runs on.

The file is useful to correctly implement the sysconf() function to return
the number of online CPUs.  In the presence of hotplug we currently cannot
provide this information.  The patch below should to it.
Signed-off-by: Ulrich Drepper <drepper@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>