linux-kernel - [PATCH 2/2] docs/core-api: memory-hotplug: add some details about locking internals

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Message-Id: <1539233897-10207-3-git-send-email-rppt@linux.ibm.com>
Date:   Thu, 11 Oct 2018 07:58:17 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Jonathan Corbet <corbet@....net>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: [PATCH 2/2] docs/core-api: memory-hotplug: add some details about locking internals

From: David Hildenbrand <david@...hat.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-david@redhat.com
Signed-off-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Pavel Tatashin <pavel.tatashin@...rosoft.com>
Reviewed-by: Rashmica Gupta <rashmica.g@...il.com>
Cc: Jonathan Corbet <corbet@....net>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Balbir Singh <bsingharora@...il.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Heiko Carstens <heiko.carstens@...ibm.com>
Cc: John Allen <jallen@...ux.vnet.ibm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Kate Stewart <kstewart@...uxfoundation.org>
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
Cc: Len Brown <lenb@...nel.org>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Mathieu Malaterre <malat@...ian.org>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Michael Neuling <mikey@...ling.org>
Cc: Nathan Fontenot <nfont@...ux.vnet.ibm.com>
Cc: Oscar Salvador <osalvador@...e.de>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Philippe Ombredanne <pombredanne@...b.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: Stephen Hemminger <sthemmin@...rosoft.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: YASUAKI ISHIMATSU <yasu.isimatu@...il.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Mike Rapoport <rppt@...ux.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