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Subject[PATCH 2/2] docs/core-api: memory-hotplug: add some details about locking internals
From: David Hildenbrand <>

Let's document the magic a bit, especially why device_hotplug_lock is
required when adding/removing memory and how it all play together with
requests to online/offline memory from user space.

[ rppt: moved the text to Documentation/core-api/memory-hotplug.rst ]

Signed-off-by: David Hildenbrand <>
Reviewed-by: Pavel Tatashin <>
Reviewed-by: Rashmica Gupta <>
Cc: Jonathan Corbet <>
Cc: Michal Hocko <>
Cc: Balbir Singh <>
Cc: Benjamin Herrenschmidt <>
Cc: Boris Ostrovsky <>
Cc: Dan Williams <>
Cc: Greg Kroah-Hartman <>
Cc: Haiyang Zhang <>
Cc: Heiko Carstens <>
Cc: John Allen <>
Cc: Joonsoo Kim <>
Cc: Juergen Gross <>
Cc: Kate Stewart <>
Cc: "K. Y. Srinivasan" <>
Cc: Len Brown <>
Cc: Martin Schwidefsky <>
Cc: Mathieu Malaterre <>
Cc: Michael Ellerman <>
Cc: Michael Neuling <>
Cc: Nathan Fontenot <>
Cc: Oscar Salvador <>
Cc: Paul Mackerras <>
Cc: Philippe Ombredanne <>
Cc: Rafael J. Wysocki <>
Cc: "Rafael J. Wysocki" <>
Cc: Stephen Hemminger <>
Cc: Thomas Gleixner <>
Cc: Vlastimil Babka <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Mike Rapoport <>
Documentation/core-api/memory-hotplug.rst | 38 +++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)

diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
index a99f2f2..de7467e 100644
--- a/Documentation/core-api/memory-hotplug.rst
+++ b/Documentation/core-api/memory-hotplug.rst
@@ -85,3 +85,41 @@ MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
further processing of the notification queue.

NOTIFY_STOP stops further processing of the notification queue.
+Locking Internals
+When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
+the device_hotplug_lock should be held to:
+- synchronize against online/offline requests (e.g. via sysfs). This way, memory
+ block devices can only be accessed (.online/.state attributes) by user
+ space once memory has been fully added. And when removing memory, we
+ know nobody is in critical sections.
+- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
+Especially, there is a possible lock inversion that is avoided using
+device_hotplug_lock when adding memory and user space tries to online that
+memory faster than expected:
+- device_online() will first take the device_lock(), followed by
+ mem_hotplug_lock
+- add_memory_resource() will first take the mem_hotplug_lock, followed by
+ the device_lock() (while creating the devices, during bus_add_device()).
+As the device is visible to user space before taking the device_lock(), this
+can result in a lock inversion.
+onlining/offlining of memory should be done via device_online()/
+device_offline() - to make sure it is properly synchronized to actions
+via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
+When adding/removing/onlining/offlining memory or adding/removing
+heterogeneous/device memory, we should always hold the mem_hotplug_lock in
+write mode to serialise memory hotplug (e.g. access to global/zone
+In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
+mode allows for a quite efficient get_online_mems/put_online_mems
+implementation, so code accessing memory can protect from that memory
 \ /
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