Add irq protection in the percpu-counters cpu-hotplug-callback path

Some of the per-cpu counters and thus their locks are accessed from IRQ
contexts.  This can cause a deadlock if it interrupts a cpu-offline thread
which is transferring a dead-cpu's counts to the global counter.

Add appropriate IRQ protection in the cpu-hotplug callback path.
Signed-off-by: NGautham R Shenoy <ego@in.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/percpu_counter.c

@@ -124,12 +124,13 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
 	mutex_lock(&percpu_counters_lock);
 	list_for_each_entry(fbc, &percpu_counters, list) {
 		s32 *pcount;
+		unsigned long flags;
 
-		spin_lock(&fbc->lock);
+		spin_lock_irqsave(&fbc->lock, flags);
 		pcount = per_cpu_ptr(fbc->counters, cpu);
 		fbc->count += *pcount;
 		*pcount = 0;
-		spin_unlock(&fbc->lock);
+		spin_unlock_irqrestore(&fbc->lock, flags);
 	}
 	mutex_unlock(&percpu_counters_lock);
 	return NOTIFY_OK;