From 3a7452c5a72bd8098f6d4b37341e25a8725d790b Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Thu, 11 Oct 2018 07:58:17 +0300 Subject: [PATCH] docs/core-api: memory-hotplug: 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. [ rppt: moved the text to Documentation/core-api/memory-hotplug.rst ] Link: http://lkml.kernel.org/r/20180925091457.28651-7-david@redhat.com 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 Cc: YASUAKI ISHIMATSU Signed-off-by: Andrew Morton Signed-off-by: Mike Rapoport Signed-off-by: Jonathan Corbet --- 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 a99f2f264725..de7467e48067 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.