On 11/10/2018 06:58, Mike Rapoport wrote: > From: David Hildenbrand <da...@redhat.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-da...@redhat.com > Signed-off-by: David Hildenbrand <da...@redhat.com> > Reviewed-by: Pavel Tatashin <pavel.tatas...@microsoft.com> > Reviewed-by: Rashmica Gupta <rashmic...@gmail.com> > Cc: Jonathan Corbet <cor...@lwn.net> > Cc: Michal Hocko <mho...@suse.com> > Cc: Balbir Singh <bsinghar...@gmail.com> > Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> > Cc: Boris Ostrovsky <boris.ostrov...@oracle.com> > Cc: Dan Williams <dan.j.willi...@intel.com> > Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> > Cc: Haiyang Zhang <haiya...@microsoft.com> > Cc: Heiko Carstens <heiko.carst...@de.ibm.com> > Cc: John Allen <jal...@linux.vnet.ibm.com> > Cc: Joonsoo Kim <iamjoonsoo....@lge.com> > Cc: Juergen Gross <jgr...@suse.com> > Cc: Kate Stewart <kstew...@linuxfoundation.org> > Cc: "K. Y. Srinivasan" <k...@microsoft.com> > Cc: Len Brown <l...@kernel.org> > Cc: Martin Schwidefsky <schwidef...@de.ibm.com> > Cc: Mathieu Malaterre <ma...@debian.org> > Cc: Michael Ellerman <m...@ellerman.id.au> > Cc: Michael Neuling <mi...@neuling.org> > Cc: Nathan Fontenot <nf...@linux.vnet.ibm.com> > Cc: Oscar Salvador <osalva...@suse.de> > Cc: Paul Mackerras <pau...@samba.org> > Cc: Philippe Ombredanne <pombreda...@nexb.com> > Cc: Rafael J. Wysocki <rafael.j.wyso...@intel.com> > Cc: "Rafael J. Wysocki" <r...@rjwysocki.net> > Cc: Stephen Hemminger <sthem...@microsoft.com> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Vlastimil Babka <vba...@suse.cz> > Cc: YASUAKI ISHIMATSU <yasu.isim...@gmail.com> > Signed-off-by: Andrew Morton <a...@linux-foundation.org> > Signed-off-by: Mike Rapoport <r...@linux.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. >
Looks good to me. -- Thanks, David / dhildenb
