> Hi Pavel,
>
> I am working on adding this sort of a workflow into a new daxctl command
> (daxctl-reconfigure-device)- this will allow changing the 'mode' of a
> dax device to kmem, online the resulting memory, and with your patches,
> also attempt to offline the memory, and change back to device-dax.
>
> In running with these patches, and testing the offlining part, I ran
> into the following lockdep below.
>
> This is with just these three patches on top of -rc7.
>
>
> [  +0.004886] ======================================================
> [  +0.001576] WARNING: possible circular locking dependency detected
> [  +0.001506] 5.1.0-rc7+ #13 Tainted: G           O
> [  +0.000929] ------------------------------------------------------
> [  +0.000708] daxctl/22950 is trying to acquire lock:
> [  +0.000548] 00000000f4d397f7 (kn->count#424){++++}, at: 
> kernfs_remove_by_name_ns+0x40/0x80
> [  +0.000922]
>               but task is already holding lock:
> [  +0.000657] 000000002aa52a9f (mem_sysfs_mutex){+.+.}, at: 
> unregister_memory_section+0x22/0xa0

I have studied this issue, and now have a clear understanding why it
happens, I am not yet sure how to fix it, so suggestions are welcomed
:)

Here is the problem:

When we offline pages we have the following call stack:

# echo offline > /sys/devices/system/memory/memory8/state
ksys_write
 vfs_write
  __vfs_write
   kernfs_fop_write
    kernfs_get_active
     lock_acquire                       kn->count#122 (lock for
"memory8/state" kn)
    sysfs_kf_write
     dev_attr_store
      state_store
       device_offline
        memory_subsys_offline
         memory_block_action
          offline_pages
           __offline_pages
            percpu_down_write
             down_write
              lock_acquire              mem_hotplug_lock.rw_sem

When we unbind dax0.0 we have the following  stack:
# echo dax0.0 > /sys/bus/dax/drivers/kmem/unbind
drv_attr_store
 unbind_store
  device_driver_detach
   device_release_driver_internal
    dev_dax_kmem_remove
     remove_memory                      device_hotplug_lock
      try_remove_memory                 mem_hotplug_lock.rw_sem
       arch_remove_memory
        __remove_pages
         __remove_section
          unregister_memory_section
           remove_memory_section        mem_sysfs_mutex
            unregister_memory
             device_unregister
              device_del
               device_remove_attrs
                sysfs_remove_groups
                 sysfs_remove_group
                  remove_files
                   kernfs_remove_by_name
                    kernfs_remove_by_name_ns
                     __kernfs_remove    kn->count#122

So, lockdep found the ordering issue with the above two stacks:

1. kn->count#122 -> mem_hotplug_lock.rw_sem
2. mem_hotplug_lock.rw_sem -> kn->count#122

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