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: David Hildenbrand <david@redhat.com> Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com> Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com> Reviewed-by: Oscar 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: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
parent
5666848774
commit
dee6da22ef
|
@ -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
|
||||
===========
|
||||
|
||||
|
|
Loading…
Reference in New Issue