[Xen-devel] [PATCH RFCv2 3/6] mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock

[Rashmica <rashmica.g@xxxxxxxxx>]

Hi David,


On 21/08/18 20:44, David Hildenbrand wrote:

There seem to be some problems as result of 30467e0b3be ("mm, hotplug:
fix concurrent memory hot-add deadlock"), which tried to fix a possible
lock inversion reported and discussed in [1] due to the two locks
	a) device_lock()
	b) mem_hotplug_lock

While add_memory() first takes b), followed by a) during
bus_probe_device(), onlining of memory from user space first took b),
followed by a), exposing a possible deadlock.

Do you mean "onlining of memory from user space first took a),
followed by b)"? 

In [1], and it was decided to not make use of device_hotplug_lock, but
rather to enforce a locking order.

The problems I spotted related to this:

1. Memory block device attributes: While .state first calls
   mem_hotplug_begin() and the calls device_online() - which takes
   device_lock() - .online does no longer call mem_hotplug_begin(), so
   effectively calls online_pages() without mem_hotplug_lock.

2. device_online() should be called under device_hotplug_lock, however
   onlining memory during add_memory() does not take care of that.

In addition, I think there is also something wrong about the locking in

3. arch/powerpc/platforms/powernv/memtrace.c calls offline_pages()
   without locks. This was introduced after 30467e0b3be. And skimming over
   the code, I assume it could need some more care in regards to locking
   (e.g. device_online() called without device_hotplug_lock - but I'll
   not touch that for now).

Can you mention that you fixed this in later patches?

The series looks good to me. Feel free to add my reviewed-by:

Reviewed-by: Rashmica Gupta <rashmica.g@xxxxxxxxx>

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