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Date:   Wed, 15 May 2019 14:11:58 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     "Verma, Vishal L" <vishal.l.verma@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "tiwai@...e.de" <tiwai@...e.de>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "david@...hat.com" <david@...hat.com>, "bp@...e.de" <bp@...e.de>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        "zwisler@...nel.org" <zwisler@...nel.org>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "Busch, Keith" <keith.busch@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Wu, Fengguang" <fengguang.wu@...el.com>,
        "baiyaowei@...s.chinamobile.com" <baiyaowei@...s.chinamobile.com>
Subject: Re: [v5 0/3] "Hotremove" persistent memory

> 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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