I/O stall since 4.19.1

I/O stall since 4.19.1

[candyangel]

Heyas,

I've discovered an issue when upgrading my sisters computer where
kernels 4.19.1 and above (up to 4.20.3, the latest I could test with)
result in intermittent stalling of the machine as each process gets
"stuck" waiting on disk I/O, even though there is no actual disk
activity occuring (no activity in iotop, HDD LED is off and the disks
are making no seek noise).

4.19.1 seems to have stalls of around 3 minutes and 4.20.3 would stall
for upwards of 20 minutes (if it ever recovered at all).

I'm unable to specifically trigger the stall, though in my testing it
manifests within 3 hours of boot during light usage (browsing the
internet, using Discord).

I logged an issue on bugzilla which contains stack trace of blocked
processes during a stall:
https://bugzilla.kernel.org/show_bug.cgi?id=202353.

As mentioned there, I can reliably start and stop the behaviour by
changing kernel versions, so I don't believe it is a hardware issue.
The disk setup is 2 SATA HDDs with RAID10f2 and ext4 partitions.

I'm happy to try and do what I can to help isolate the issue, but please
bear in mind that I don't have much experience debugging kernel issues!

Thanks for your time,
CandyAngel