From: David Hildenbrand <da...@redhat.com> 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 ] Link: http://lkml.kernel.org/r/20180925091457.28651-7-da...@redhat.com Signed-off-by: David Hildenbrand <da...@redhat.com> Reviewed-by: Pavel Tatashin <pavel.tatas...@microsoft.com> Reviewed-by: Rashmica Gupta <rashmic...@gmail.com> Cc: Jonathan Corbet <cor...@lwn.net> Cc: Michal Hocko <mho...@suse.com> Cc: Balbir Singh <bsinghar...@gmail.com> Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> Cc: Boris Ostrovsky <boris.ostrov...@oracle.com> Cc: Dan Williams <dan.j.willi...@intel.com> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> Cc: Haiyang Zhang <haiya...@microsoft.com> Cc: Heiko Carstens <heiko.carst...@de.ibm.com> Cc: John Allen <jal...@linux.vnet.ibm.com> Cc: Joonsoo Kim <iamjoonsoo....@lge.com> Cc: Juergen Gross <jgr...@suse.com> Cc: Kate Stewart <kstew...@linuxfoundation.org> Cc: "K. Y. Srinivasan" <k...@microsoft.com> Cc: Len Brown <l...@kernel.org> Cc: Martin Schwidefsky <schwidef...@de.ibm.com> Cc: Mathieu Malaterre <ma...@debian.org> Cc: Michael Ellerman <m...@ellerman.id.au> Cc: Michael Neuling <mi...@neuling.org> Cc: Nathan Fontenot <nf...@linux.vnet.ibm.com> Cc: Oscar Salvador <osalva...@suse.de> Cc: Paul Mackerras <pau...@samba.org> Cc: Philippe Ombredanne <pombreda...@nexb.com> Cc: Rafael J. Wysocki <rafael.j.wyso...@intel.com> Cc: "Rafael J. Wysocki" <r...@rjwysocki.net> Cc: Stephen Hemminger <sthem...@microsoft.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Vlastimil Babka <vba...@suse.cz> Cc: YASUAKI ISHIMATSU <yasu.isim...@gmail.com> Signed-off-by: Andrew Morton <a...@linux-foundation.org> Signed-off-by: Mike Rapoport <r...@linux.ibm.com> --- 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 +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. -- 2.7.4
