From: David Hildenbrand <[email protected]> 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/[email protected] Signed-off-by: David Hildenbrand <[email protected]> Reviewed-by: Pavel Tatashin <[email protected]> Reviewed-by: Rashmica Gupta <[email protected]> Cc: Jonathan Corbet <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Balbir Singh <[email protected]> Cc: Benjamin Herrenschmidt <[email protected]> Cc: Boris Ostrovsky <[email protected]> Cc: Dan Williams <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> Cc: Haiyang Zhang <[email protected]> Cc: Heiko Carstens <[email protected]> Cc: John Allen <[email protected]> Cc: Joonsoo Kim <[email protected]> Cc: Juergen Gross <[email protected]> Cc: Kate Stewart <[email protected]> Cc: "K. Y. Srinivasan" <[email protected]> Cc: Len Brown <[email protected]> Cc: Martin Schwidefsky <[email protected]> Cc: Mathieu Malaterre <[email protected]> Cc: Michael Ellerman <[email protected]> Cc: Michael Neuling <[email protected]> Cc: Nathan Fontenot <[email protected]> Cc: Oscar Salvador <[email protected]> Cc: Paul Mackerras <[email protected]> Cc: Philippe Ombredanne <[email protected]> Cc: Rafael J. Wysocki <[email protected]> Cc: "Rafael J. Wysocki" <[email protected]> Cc: Stephen Hemminger <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Vlastimil Babka <[email protected]> Cc: YASUAKI ISHIMATSU <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Mike Rapoport <[email protected]> --- 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
