current/netbsd-10: panic() when MULTIPROCESSOR is disabled

I'm slimming down a GENERIC i386 kernel[1] for an embedded board[2], when i
discovered that turning off MULTIPROCESSOR, leads to:

[  12.3802131] Mutex error: mutex_spin_retry,979: locking against myself
...
[  12.3802131] panic: lock error: Mutex: mutex_spin_retry,979: locking
against myself: lock 0xc1052b08 cpu 0 lwp 0xc0df7300
...

here the full trace: https://dpaste.org/ZsVvf

$ gdb ../obj.i386/sys/arch/i386/compile/ALIX/netbsd.gdb
(gdb) info line *(knote+0x34)
Line 2937 of "/home/flag/bsd/netbsd/src/sys/kern/kern_event.c" starts at
address 0xc03bec0c <knote+52> and ends at 0xc03bec13 <knote+59>.

so, if i'm reading it correctly:

static void
knote_activate(struct knote *kn) {

struct kqueue *kq = kn->kn_kq;

mutex_spin_enter(&kq->kq_lock); <--- is it this one???
knote_activate_locked(kn);            <--- 2937 is actually this line,
though
mutex_spin_exit(&kq->kq_lock);
}

the annoying part is that adding any debug option (e.g. LOCKDEBUG, etc)
makes the panic disappear.

Any idea how can i debug it further?


1: https://dpaste.com//GM8KQV74X
2: https://www.pcengines.ch/alix.htm


-- 
bye,
p.