The patch hasn't made it into mainline from what I have seen, it looks
like it died back in March waiting for feedback from additional kernel
developers.  From what I have gathered this is a deadlock scenario
directly caused by pausing the resync while the system is under heavy
write activity.

Donald Buczek provided a reproducer which is a shell script that
generates a lot of write activity and pauses/resumes the raid scrubbing.
And he also provided a workaround to get the stuck system running
without reboot:

echo active > /sys/block/md1/md/array_state


I haven't tried the patch or any of this, I pretty much eliminated the trigger 
which is mdcheck_start & mdcheck_continue and went back to the 18.04 LTS way of 
scrubbing arrays (which is basically don't pause/interrupt it once it starts).  
I ran through a checkarray yesterday to 100%, no problems.  Meanwhile since 
upgrading to 20.04 LTS it has hung almost every single time through 5.4, 5.8, 
and 5.11 kernels.

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https://bugs.launchpad.net/bugs/1942935

Title:
  kernel io hangs during mdcheck/resync

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