tick/nohz: Prevent stopping the tick on an offline CPU

can_stop_full_tick() has no check for offline cpus. So it allows to stop
the tick on an offline cpu from the interrupt return path, which is wrong
and subsequently makes irq_work_needs_cpu() warn about being called for an
offline cpu.

Commit f7ea0fd639c2c4 ("tick: Don't invoke tick_nohz_stop_sched_tick() if
the cpu is offline") added prevention for can_stop_idle_tick(), but forgot
to do the same in can_stop_full_tick(). Add it.

[ tglx: Massaged changelog ]

Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1473245473-4463-1-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/tick-sched.c

@@ -186,10 +186,13 @@ static bool check_tick_dependency(atomic_t *dep)
 	return false;
 }
 
-static bool can_stop_full_tick(struct tick_sched *ts)
+static bool can_stop_full_tick(int cpu, struct tick_sched *ts)
 {
 	WARN_ON_ONCE(!irqs_disabled());
 
+	if (unlikely(!cpu_online(cpu)))
+		return false;
+
 	if (check_tick_dependency(&tick_dep_mask))
 		return false;
 
@@ -843,7 +846,7 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
 	if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
 		return;
 
-	if (can_stop_full_tick(ts))
+	if (can_stop_full_tick(cpu, ts))
 		tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
 	else if (ts->tick_stopped)
 		tick_nohz_restart_sched_tick(ts, ktime_get());