On Tue, Jan 28, 2020 at 12:11 AM Georgi Nikolov [off-list ref] wrote:

Yes the kernel is 4.19.67-2+deb10u2. I have tried with 5.4.8-1~bpo10+1
and same thing happened.
Before this i have used same thing for a long time with kernel 4.9.189-3.
It happens most often when there is some heavy IO.

Thanks for the information. I suspect the hang is waiting for md_lock().

Could you please dump the mdX_raid6 stack when the hang happens?
You can do it by:

    cat /proc/$(pidof mdX_raid6)/stack

Thanks,
Song