On 11/10/2018 06:58, Mike Rapoport wrote: > 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. >
Looks good to me. -- Thanks, David / dhildenb
