Hi,

I got kernel panic since v5.1-rc1 when working with files on block
device with BFQ scheduler assigned. I didn't find trivial way to
reproduce the panic but "git checkout origin/linux-5.0.y"
on linux-stable-rc[1] git repo on btrfs filesystem reproduces the
problem 100% of the time on my bare-metal machine and in a VM.

Bisect led me to commit 9dee8b3b057e1 (block, bfq: fix queue removal
from weights tree). After reverting this commit on top of current
mainline master(9936328b41ce) I can't reproduce the problem.

dmesg with the panic and bisect log attached.

[1] https://kernel.googlesource.com/pub/scm/linux/kernel/git/stable/linux-stable-rc.git