Fix stop_machine_run problem with naughty real time process

stop_machine_run() does its work on "kstopmachine" thread having max
priority.  However that thread get such priority after woken up.
Therefore, in the following case ...

  - "kstopmachine" try to run on CPU1

  - There is a real time process which doesn't relinquish CPU time
    voluntary on CPU1

...  "kstopmachine" can't start to run and the CPU on which
    stop_machine_run() is runing hangs up.  To fix this problem, call
    sched_setscheduler() before waking up that thread.
Signed-off-by: NSatoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/stop_machine.c

@@ -93,10 +93,6 @@ static void stopmachine_set_state(enum stopmachine_state state)
 static int stop_machine(void)
 {
 	int i, ret = 0;
-	struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
-
-	/* One high-prio thread per cpu.  We'll do this one. */
-	sched_setscheduler(current, SCHED_FIFO, &param);
 
 	atomic_set(&stopmachine_thread_ack, 0);
 	stopmachine_num_threads = 0;
@@ -189,6 +185,10 @@ struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
 
 	p = kthread_create(do_stop, &smdata, "kstopmachine");
 	if (!IS_ERR(p)) {
+		struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
+
+		/* One high-prio thread per cpu.  We'll do this one. */
+		sched_setscheduler(p, SCHED_FIFO, &param);
 		kthread_bind(p, cpu);
 		wake_up_process(p);
 		wait_for_completion(&smdata.done);