Comment 8 for bug 1837788

Tested with bionic-proposed + patch (problem does not happen)
---

Test kernel available in:
  https://people.canonical.com/~mfo/lp1837788/linux_4.15.0-56.62+test20190725b1_amd64/

# uname -rv
4.15.0-56-generic #62+test20190725b1 SMP Thu Jul 25 12:44:16 -03 2019

# ./setup-bcache-wb_percent-before-attach.sh >/dev/null 2>&1
#
[ 61.772497] bcache: register_bdev() registered backing device loop0/null 2>&1
[ 61.789479] bcache: run_cache_set() invalidating existing data
[ 61.799599] bcache: register_cache() registered cache device loop1
[ 65.772621] bcache: bch_cached_dev_attach() Caching loop0 as bcache0 on set 2c09c59b-35b1-4da7-8225-d143c1ea81d2

# reboot
# # comment last line in script.

# ./setup-bcache-wb_percent-before-attach.sh >/dev/null 2>&1
#
[ 36.853197] bcache: register_bdev() registered backing device loop0/null 2>&1
[ 36.885302] bcache: run_cache_set() invalidating existing data
[ 36.896425] bcache: register_cache() registered cache device loop1
(wait a few seconds)
(ok no oops anymore)