I get this error (trivially reproducible) on port-evbarm/TS7200 kernels,
both 4.99.16 from a couple of days ago and 4.99.17 built ~24h ago.

The lock in question is no other than sched_mutex in kern_synch.o, which 
seems pretty bad. :-)

Interestingly enough, the problem disappears if the kernel is compiled 
with options LOCKDEBUG. It seems evbarm usage of this option is isolated 
to arm32/pmap.c, so I guess I'll continue looking there...

Anyone else seen this on other platforms?

.oO( just fishing to see if this is exclusively evbarm, or MI )

Should I grab the panic message and ddb bt/ps output and file a PR with 
the details?

Best regards,
ali:)