slab: shut down cache_reaper when cpu goes down

Shutdown the cache_reaper if the cpu is brought down and set the
cache_reap.func to NULL.  Otherwise hotplug shuts down the reaper for good.
Signed-off-by: NChristoph Lameter <clameter@sgi.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

@@ -1279,6 +1279,20 @@ static int __cpuinit cpuup_callback(struct notifier_block *nfb,
 		start_cpu_timer(cpu);
 		break;
 #ifdef CONFIG_HOTPLUG_CPU
+  	case CPU_DOWN_PREPARE:
+		/*
+		 * Shutdown cache reaper. Note that the cache_chain_mutex is
+		 * held so that if cache_reap() is invoked it cannot do
+		 * anything expensive but will only modify reap_work
+		 * and reschedule the timer.
+		*/
+		cancel_rearming_delayed_work(&per_cpu(reap_work, cpu));
+		/* Now the cache_reaper is guaranteed to be not running. */
+		per_cpu(reap_work, cpu).work.func = NULL;
+  		break;
+  	case CPU_DOWN_FAILED:
+		start_cpu_timer(cpu);
+  		break;
 	case CPU_DEAD:
 		/*
 		 * Even if all the cpus of a node are down, we don't free the