[PATCH 2/3] memory_hotplug: add note about holding device_hotplug_lock and add_memory()

[Vitaly Kuznetsov <vkuznets@redhat.com>]

add_memory() is supposed to be run with device_hotplug_lock grabbed, otherwise
it can race with e.g. device_online(). ACPI memory hotplug does that already
but e.g. Hyper-V ballooning driver doesn't.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 mm/memory_hotplug.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 9fab107..41638eb 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1213,7 +1213,11 @@ int zone_for_memory(int nid, u64 start, u64 size, int zone_default)
 	return zone_default;
 }
 
-/* we are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG */
+/*
+ * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
+ * and online/offline operations before this call.
+ * We are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG.
+ */
 int __ref add_memory(int nid, u64 start, u64 size)
 {
 	pg_data_t *pgdat = NULL;
-- 
1.9.3