powerpc/sysfs: Fix reference leak of cpu device_nodes present at boot

commit e76ca277 upstream.

For CPUs present at boot each logical CPU acquires a reference to the
associated device node of the core. This happens in register_cpu() which
is called by topology_init(). The result of this is that we end up with
a reference held by each thread of the core. However, these references
are never freed if the CPU core is DLPAR removed.

This patch fixes the reference leaks by acquiring and releasing the references
in the CPU hotplug callbacks un/register_cpu_online(). With this patch symmetric
reference counting is observed with both CPUs present at boot, and those DLPAR
added after boot.

Fixes: f86e4718 ("driver/core: cpu: initialize of_node in cpu's device struture")
Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/powerpc/kernel/sysfs.c

@@ -672,6 +672,10 @@ static void register_cpu_online(unsigned int cpu)
 	struct device_attribute *attrs, *pmc_attrs;
 	int i, nattrs;
 
+	/* For cpus present at boot a reference was already grabbed in register_cpu() */
+	if (!s->of_node)
+		s->of_node = of_get_cpu_node(cpu, NULL);
+
 #ifdef CONFIG_PPC64
 	if (cpu_has_feature(CPU_FTR_SMT))
 		device_create_file(s, &dev_attr_smt_snooze_delay);
@@ -825,6 +829,8 @@ static void unregister_cpu_online(unsigned int cpu)
 	}
 #endif
 	cacheinfo_cpu_offline(cpu);
+	of_node_put(s->of_node);
+	s->of_node = NULL;
 }
 
 #ifdef CONFIG_ARCH_CPU_PROBE_RELEASE