:Updated: Add some details about locking internals: Aug 20 2018

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

variables).

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

vanishing.