Commit dee6da22 authored by David Hildenbrand's avatar David Hildenbrand Committed by Linus Torvalds
Browse files

memory-hotplug.rst: add some details about locking internals 

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.

Link: http://lkml.kernel.org/r/20180925091457.28651-7-david@redhat.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarPavel Tatashin <pavel.tatashin@microsoft.com>
Reviewed-by: default avatarRashmica Gupta <rashmica.g@gmail.com>
Reviewed-by: default avatarOscar Salvador <osalvador@suse.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: John Allen <jallen@linux.vnet.ibm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Mathieu Malaterre <malat@debian.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michael Neuling <mikey@neuling.org>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 56668487

Documentation/admin-guide/mm/memory-hotplug.rst

+41 −1
Original line number Diff line number Diff line
@@ -5,7 +5,7 @@ Memory Hotplug 
==============



:Created:							Jul 28 2007

:Updated: Add description of notifier of memory hotplug:	Oct 11 2007

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



This document is about memory hotplug including how-to-use and current status.

Because Memory Hotplug is still under development, contents of this text will
@@ -392,6 +392,46 @@ Need more implementation yet.... 
 - Notification completion of remove works by OS to firmware.

 - Guard from remove if not yet.





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.





Future Work

===========

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