the end.

It has two problems:

1) The recovery of the current tasks's cpus_allowed will fail under
   some conditions.

   # grep Cpus_allowed_list /proc/$$/status
   Cpus_allowed_list:      0-3

   # taskset -pc 2 $$
   pid 29075's current affinity list: 0-3
   pid 29075's new affinity list: 2

   # grep Cpus_allowed_list /proc/$$/status
   Cpus_allowed_list:      2

   # echo 0 > /sys/devices/system/cpu/cpu2/online

   # grep Cpus_allowed_list /proc/$$/status
   Cpus_allowed_list:      0

   Here, the Cpus_allowed_list was originally "2" and has become
   "0-1,3" after cpu #2 is offlined.

   This "Cpus_allowed_list:      0" is incorrect.

2) If the current task is a userspace task, the user may change its
   cpu-affinity during the CPU hot-unplugging.  This change can be
   overwritten when _cpu_down() changes the current task's affinity.


Fix all this by not changing the current tasks's affinity.  Instead we
create a kernel thread to do the work.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>