From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Hildenbrand Subject: [PATCH RFCv2 6/6] memory-hotplug.txt: Add some details about locking internals Date: Tue, 21 Aug 2018 12:44:18 +0200 Message-ID: <20180821104418.12710-7-david@redhat.com> References: <20180821104418.12710-1-david@redhat.com> Return-path: In-Reply-To: <20180821104418.12710-1-david@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-acpi@vger.kernel.org, xen-devel@lists.xenproject.org, devel@linuxdriverproject.org, David Hildenbrand , Jonathan Corbet , Michal Hocko , Andrew Morton List-Id: linux-acpi@vger.kernel.org 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. Cc: Jonathan Corbet Cc: Michal Hocko Cc: Andrew Morton Signed-off-by: David Hildenbrand --- Documentation/memory-hotplug.txt | 39 +++++++++++++++++++++++++++++++- 1 file changed, 38 insertions(+), 1 deletion(-) diff --git a/Documentation/memory-hotplug.txt b/Documentation/memory-hotplug.txt index 7f49ebf3ddb2..03aaad7d7373 100644 --- a/Documentation/memory-hotplug.txt +++ b/Documentation/memory-hotplug.txt @@ -3,7 +3,7 @@ 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 @@ -495,6 +495,43 @@ 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 to +serialise memory hotplug (e.g. access to global/zone variables). + +In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) allows +for a quite efficient get_online_mems/put_online_mems implementation. + + Future Work =========== -- 2.17.1