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

[David Hildenbrand <david@redhat.com>]

On 11/10/2018 06:58, Mike Rapoport wrote:
> From: David Hildenbrand <david@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-david@redhat.com
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>
> Reviewed-by: Rashmica Gupta <rashmica.g@gmail.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Balbir Singh <bsingharora@gmail.com>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: John Allen <jallen@linux.vnet.ibm.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: Len Brown <lenb@kernel.org>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Mathieu Malaterre <malat@debian.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Michael Neuling <mikey@neuling.org>
> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Philippe Ombredanne <pombredanne@nexb.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Mike Rapoport <rppt@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