sched: do not use cpu_to_node() to find an offlined cpu's node.

If a cpu is offline, its nid will be set to -1, and cpu_to_node(cpu)
will return -1.  As a result, cpumask_of_node(nid) will return NULL.  In
this case, find_next_bit() in for_each_cpu will get a NULL pointer and
cause panic.

Here is a call trace:
  Call Trace:
   <IRQ>
    select_fallback_rq+0x71/0x190
    try_to_wake_up+0x2cb/0x2f0
    wake_up_process+0x15/0x20
    hrtimer_wakeup+0x22/0x30
    __run_hrtimer+0x83/0x320
    hrtimer_interrupt+0x106/0x280
    smp_apic_timer_interrupt+0x69/0x99
    apic_timer_interrupt+0x6f/0x80

There is a hrtimer process sleeping, whose cpu has already been
offlined.  When it is waken up, it tries to find another cpu to run, and
get a -1 nid.  As a result, cpumask_of_node(-1) returns NULL, and causes
ernel panic.

This patch fixes this problem by judging if the nid is -1.  If nid is
not -1, a cpu on the same node will be picked.  Else, a online cpu on
another node will be picked.
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sched/core.c

@@ -1132,10 +1132,19 @@ EXPORT_SYMBOL_GPL(kick_process);
  */
 static int select_fallback_rq(int cpu, struct task_struct *p)
 {
-	const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(cpu));
+	int nid = cpu_to_node(cpu);
+	const struct cpumask *nodemask = NULL;
 	enum { cpuset, possible, fail } state = cpuset;
 	int dest_cpu;
 
+	/*
+	 * If the node that the cpu is on has been offlined, cpu_to_node()
+	 * will return -1. There is no cpu on the node, and we should
+	 * select the cpu on the other node.
+	 */
+	if (nid != -1) {
+		nodemask = cpumask_of_node(nid);
+
 		/* Look for allowed, online CPU in same node. */
 		for_each_cpu(dest_cpu, nodemask) {
 			if (!cpu_online(dest_cpu))
@@ -1145,6 +1154,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 			if (cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)))
 				return dest_cpu;
 		}
+	}
 
 	for (;;) {
 		/* Any allowed, online CPU? */