powerpc/nohash: Allocate stale_map[cpu] on CPU_UP_PREPARE not CPU_ONLINE

Currently we allocate the stale_map for a cpu when it comes online,
this leaves open a small window where a process can be scheduled
on the cpu before the stale_map is allocated. Instead allocate
the stale_map at CPU_UP_PREPARE time, that way it will be always
available before tasks start running.

It is possible the cpu fails to come up, in which case we should free
the stale_map, so add a CPU_UP_CANCELED case to do that.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/mmu_context_nohash.c

@@ -338,12 +338,14 @@ static int __cpuinit mmu_context_cpu_notify(struct notifier_block *self,
 		return NOTIFY_OK;
 
 	switch (action) {
-	case CPU_ONLINE:
-	case CPU_ONLINE_FROZEN:
+	case CPU_UP_PREPARE:
+	case CPU_UP_PREPARE_FROZEN:
 		pr_devel("MMU: Allocating stale context map for CPU %d\n", cpu);
 		stale_map[cpu] = kzalloc(CTX_MAP_SIZE, GFP_KERNEL);
 		break;
 #ifdef CONFIG_HOTPLUG_CPU
+	case CPU_UP_CANCELED:
+	case CPU_UP_CANCELED_FROZEN:
 	case CPU_DEAD:
 	case CPU_DEAD_FROZEN:
 		pr_devel("MMU: Freeing stale context map for CPU %d\n", cpu);