powerpc/pseries: Fix cpu hotplug crash with memoryless nodes

On powerpc systems with shared configurations of CPUs and memory and
memoryless nodes at boot, an event ordering problem was observed on a
SLES12 build platforms with the hot-add of CPUs to the memoryless
nodes.

* The most common error occurred when the memory SLAB driver attempted
  to reference the memoryless node to which a CPU was being added
  before the kernel had finished initializing all of the data
  structures for the CPU and exited 'device_online' under
  DLPAR/hot-add.

  Normally the memoryless node would be initialized through the call
  path device_online ... arch_update_cpu_topology ... find_cpu_nid ...
  try_online_node. This patch ensures that the powerpc node will be
  initialized as early as possible, even if it was memoryless and
  CPU-less at the point when we are trying to hot-add a new CPU to it.

Signed-off-by: Michael Bringmann <mwb@linux.vnet.ibm.com>
Reviewed-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/numa.c

@@ -1198,7 +1198,7 @@ static long vphn_get_associativity(unsigned long cpu,
 	return rc;
 }
 
-static inline int find_and_online_cpu_nid(int cpu)
+int find_and_online_cpu_nid(int cpu)
 {
 	__be32 associativity[VPHN_ASSOC_BUFSIZE] = {0};
 	int new_nid;
@@ -1229,6 +1229,8 @@ static inline int find_and_online_cpu_nid(int cpu)
 #endif
 	}
 
+	pr_debug("%s:%d cpu %d nid %d\n", __FUNCTION__, __LINE__,
+		cpu, new_nid);
 	return new_nid;
 }
 

arch/powerpc/platforms/pseries/hotplug-cpu.c

@@ -340,6 +340,8 @@ static void pseries_remove_processor(struct device_node *np)
 	cpu_maps_update_done();
 }
 
+extern int find_and_online_cpu_nid(int cpu);
+
 static int dlpar_online_cpu(struct device_node *dn)
 {
 	int rc = 0;
@@ -364,6 +366,7 @@ static int dlpar_online_cpu(struct device_node *dn)
 					!= CPU_STATE_OFFLINE);
 			cpu_maps_update_done();
 			timed_topology_update(1);
+			find_and_online_cpu_nid(cpu);
 			rc = device_online(get_cpu_device(cpu));
 			if (rc)
 				goto out;