many cases, see below examples:

1: hotplug_cfd() can free cfd->cpumask, the system will crash if the
   cpu's cfd still in the call_function list:


      CPU A:                         CPU B

 smp_call_function_many()	    ......
   cpu_down()                      ......
  hotplug_cfd() ->                 ......
 free_cpumask_var(cfd->cpumask)  (receive function IPI interrupte)
                                /* read cfd->cpumask */
                          generic_smp_call_function_interrupt() ->
                         cpumask_test_and_clear_cpu(cpu, data->cpumask)

                         	CRASH!!!

2: It's not handle call_function list when cpu down, It's will lead to
   dead-wait if other path is waiting this cpu to execute function

    CPU A:                           CPU B

 smp_call_function_many(wait=0)
        ......			    CPU B down
   smp_call_function_many() -->  (cpu down before recevie function
    csd_lock(&data->csd);         IPI interrupte)

    DEAD-WAIT!!!!

  So, CPU A will dead-wait in csd_lock(), the same as
  smp_call_function_single()
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>