SYSCTL: Add a mutex to the page_alloc zone order sysctl

The zone list code clearly cannot tolerate concurrent writers (I couldn't
find any locks for that), so simply add a global mutex. No need for RCU
in this case.
Signed-off-by: Andi Kleen <ak@linux.intel.com>

mm/page_alloc.c

@@ -2402,13 +2402,14 @@ int numa_zonelist_order_handler(ctl_table *table, int write,
 {
 	char saved_string[NUMA_ZONELIST_ORDER_LEN];
 	int ret;
+	static DEFINE_MUTEX(zl_order_mutex);
 
+	mutex_lock(&zl_order_mutex);
 	if (write)
-		strncpy(saved_string, (char*)table->data,
-			NUMA_ZONELIST_ORDER_LEN);
+		strcpy(saved_string, (char*)table->data);
 	ret = proc_dostring(table, write, buffer, length, ppos);
 	if (ret)
-		return ret;
+		goto out;
 	if (write) {
 		int oldval = user_zonelist_order;
 		if (__parse_numa_zonelist_order((char*)table->data)) {
@@ -2421,7 +2422,9 @@ int numa_zonelist_order_handler(ctl_table *table, int write,
 		} else if (oldval != user_zonelist_order)
 			build_all_zonelists();
 	}
-	return 0;
+out:
+	mutex_unlock(&zl_order_mutex);
+	return ret;
 }