On 8/21/18 6:44 AM, 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.

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).

Now that we hold the device_hotplug_lock when
- adding memory (e.g. via add_memory()/add_memory_resource())
- removing memory (e.g. via remove_memory())
- device_online()/device_offline()

We can move mem_hotplug_lock usage back into
online_pages()/offline_pages().

Why is mem_hotplug_lock still needed? Essentially to make
get_online_mems()/put_online_mems() be very fast (relying on
device_hotplug_lock would be very slow), and to serialize against
addition of memory that does not create memory block devices (hmm).

[1] http://driverdev.linuxdriverproject.org/pipermail/ driverdev-devel/
    2015-February/065324.html

This patch is partly based on a patch by Vitaly Kuznetsov.

Reviewed-by: Pavel Tatashin <redacted>