zram hot_add device busy

[Sami Kerola]

Hello,

While going through if there are new util-linux bugs reported I came a
cross this https://bugs.launchpad.net/ubuntu/+source/util-linux/+bug/1645846

Simple way to reproduce the issue is:
d=$(cat /sys/class/zram-control/hot_add) && zramctl --size 256M /dev/zram$d

I am not entirely sure, but drivers/block/zram/zram_drv.c function
zram_add() should block until the device is usable. Looking the code
that it might be the device_add_disk() from block/genhd.c that should
do the blocking. But perhaps it's best if I leave such detail to
people who know the code a bit better.

One thing annoys me. I expected 'zramctl --find --size 256M' to suffer
from same issue but it does not. I can only reproduce the issue when
triggering hot_add separately, and as quick as possibly using the
path. Notice that sometimes it takes second try before the hot_add and
use triggers the issue. That is almost certainly down to speed the
system in hand, e.g., quicker the computer less likely to trigger.

-- 
Sami Kerola
http://www.iki.fi/kerolasa/