patches for stop_machine

Hi Rusty and all,

This is a proposal of minor improvement for kernel/stop_machine.c

[PATCH 1/3] stop_machine: short exit path for if we cannot create enough threads [PATCH 2/3] stop_machine: add timeout for child thread deployment [PATCH 3/3] stop_machine: add stopmachine_timeout sysctl entry

The main topic is "how about adding timeout for stop_machine?" I think it will act as a safety net.

For example (of silly situation), system can hung with following way:

  # ./silly.sh
  run an evil loop task on AP
   pid 6138's current affinity mask: ff
   pid 6138's new affinity mask: fe
   to pretend lock up, chrt -f -p 99 6138
   loop[6138] is on CPU #4
  to do stopmachine, try to off #7
  echo 0 > /sys/devices/system/cpu/cpu7/online
  (never return)

After applying patch set here, it can be prevented.

  # ./silly.sh
   :
  echo 0 > /sys/devices/system/cpu/cpu7/online
  stopmachine: Failed to stop machine in time(5s). Are there any CPUs on file?
  ./silly.sh: line 22: echo: write error: Device or resource busy
  offline is failed
  OK, kill evil loop[6138]
  try to off #7 again
  echo 0 > /sys/devices/system/cpu/cpu7/online
  CPU #7 is now offline
  done!

Please refer description of each patch for the detail. All comments are welcomed.

Thanks,
H.Seto