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Was trying to test the 'lvconvert --uncache' feature.  I created a cache LV and killed the device under the cache-pool.  After that, no LVM command would work.

[root@bp-01 ~]# lvs -a -o name,size,attr,devices vg
  LV                LSize   Attr       Devices
  [cachepool]         5.00g Cwi---C--- cachepool_cdata(0)
  [cachepool_cdata]   5.00g Cwi-ao---- /dev/sdb1(128004)
  [cachepool_cmeta]   8.00m ewi-ao---- /dev/sdb1(128002)
  lv                500.00g Cwi-a-C--- lv_corig(0)
  [lv_corig]        500.00g owi-aoC--- /dev/sdb1(0)
  [lvol0_pmspare]     8.00m ewi------- /dev/sdb1(128000)

I use the following command to disable the cache device (/dev/sdb1):
# echo offline > /sys/block/$dev/device/state

I will upload the 'vvvv' trace I got after attempting 'lvconvert --uncache' at this point.

Created attachment 1204883 [details]
Output from hung 'lvconvert --uncache vg/lv -vvvv'

I stupidly allowed the origin and cachepool to be created on the same PV in the earlier example.  Here it is again after putting them on separate devices:

[root@bp-01 ~]# !lvs
lvs -a -o name,size,attr,devices vg
  LV                LSize   Attr       Devices
  [cachepool]         5.00g Cwi---C--- cachepool_cdata(0)
  [cachepool_cdata]   5.00g Cwi-ao---- /dev/sdc1(4)
  [cachepool_cmeta]   8.00m ewi-ao---- /dev/sdc1(2)
  lv                500.00g Cwi-a-C--- lv_corig(0)
  [lv_corig]        500.00g owi-aoC--- /dev/sdb1(0)
  [lvol0_pmspare]     8.00m ewi------- /dev/sdc1(0)
[root@bp-01 ~]# off.sh /dev/sdc1
Turning off sdc

Created attachment 1204891 [details]
Output from hung 'lvconvert --uncache vg/lv -vvvv'

Created attachment 1204945 [details]
Backtrace of the stuck process (stuck on suspend ioctl)

*** This bug has been marked as a duplicate of bug 1377850 ***