On Wed, Sep 19, 2018 at 09:35:07AM +0200, David Hildenbrand wrote:

Am 19.09.18 um 03:22 schrieb Balbir Singh:

quoted

On Tue, Sep 18, 2018 at 01:48:16PM +0200, David Hildenbrand wrote:

quoted

Reading through the code and studying how mem_hotplug_lock is to be used,
I noticed that there are two places where we can end up calling
device_online()/device_offline() - online_pages()/offline_pages() without
the mem_hotplug_lock. And there are other places where we call
device_online()/device_offline() without the device_hotplug_lock.

While e.g.
	echo "online" > /sys/devices/system/memory/memory9/state
is fine, e.g.
	echo 1 > /sys/devices/system/memory/memory9/online
Will not take the mem_hotplug_lock. However the device_lock() and
device_hotplug_lock.

E.g. via memory_probe_store(), we can end up calling
add_memory()->online_pages() without the device_hotplug_lock. So we can
have concurrent callers in online_pages(). We e.g. touch in online_pages()
basically unprotected zone->present_pages then.

Looks like there is a longer history to that (see Patch #2 for details),
and fixing it to work the way it was intended is not really possible. We
would e.g. have to take the mem_hotplug_lock in device/base/core.c, which
sounds wrong.

Summary: We had a lock inversion on mem_hotplug_lock and device_lock().
More details can be found in patch 3 and patch 6.

I propose the general rules (documentation added in patch 6):

1. add_memory/add_memory_resource() must only be called with
   device_hotplug_lock.
2. remove_memory() must only be called with device_hotplug_lock. This is
   already documented and holds for all callers.
3. device_online()/device_offline() must only be called with
   device_hotplug_lock. This is already documented and true for now in core
   code. Other callers (related to memory hotplug) have to be fixed up.
4. mem_hotplug_lock is taken inside of add_memory/remove_memory/
   online_pages/offline_pages.

To me, this looks way cleaner than what we have right now (and easier to
verify). And looking at the documentation of remove_memory, using
lock_device_hotplug also for add_memory() feels natural.

That seems reasonable, but also implies that device_online() would hold
back add/remove memory, could you please also document what mode
read/write the locks need to be held? For example can the device_hotplug_lock
be held in read mode while add/remove memory via (mem_hotplug_lock) is held
in write mode?

device_hotplug_lock is an ordinary mutex. So no option there.

Only mem_hotplug_lock is a per CPU RW mutex. And as of now it only
exists to not require get_online_mems()/put_online_mems() to take the
device_hotplug_lock. Which is perfectly valid, because these users only
care about memory (not any other devices) not suddenly vanish. And that
RW lock makes things fast.

Any modifications (online/offline/add/remove) require the
mem_hotplug_lock in write.

I can add some more details to documentation in patch #6.

"... we should always hold the mem_hotplug_lock (via
mem_hotplug_begin/mem_hotplug_done) in write mode to serialize memory
hotplug" ..."

"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."

Would that work for you?

Yes, Thanks

Balbir Singh.