slub: Avoid use of slub_lock in show_slab_objects()

commit 04d94879 upstream.

The purpose of the locking is to prevent removal and additions
of nodes when statistics are gathered for a slab cache. So we
need to avoid racing with memory hotplug functionality.

It is enough to take the memory hotplug locks there instead
of the slub_lock.

online_pages() currently does not acquire the memory_hotplug
lock. Another patch will be submitted by the memory hotplug
authors to take the memory hotplug lock and describe the
uses of the memory hotplug lock to protect against
adding and removal of nodes from non hotplug data structures.
Reported-and-tested-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

mm/slub.c

@@ -3821,7 +3821,7 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
 		}
 	}
 
-	down_read(&slub_lock);
+	lock_memory_hotplug();
 #ifdef CONFIG_SLUB_DEBUG
 	if (flags & SO_ALL) {
 		for_each_node_state(node, N_NORMAL_MEMORY) {
@@ -3862,7 +3862,7 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
 			x += sprintf(buf + x, " N%d=%lu",
 					node, nodes[node]);
 #endif
-	up_read(&slub_lock);
+	unlock_memory_hotplug();
 	kfree(nodes);
 	return x + sprintf(buf + x, "\n");
 }