panic: _mtx_lock_sleep: recursed on non-recursive mutex pmap _at_ /usr/src/sys/i386/i386/pmap.c:1843

folks
booting panic with advantech's PCM-5823 board on CURRENT
appreciate with any help

snip...
Timecounter "TSC" frequency 299238514 Hz quality 800
Timecounters tick every 1.000 msec
rr232x: no controller detected.
ad2: 488MB <SanDisk SDCFB-512 HDX 2.27> at ata1-master PIO4
Trying to mount root from ufs:/dev/ad2s1a
lock order reversal:
1st 0xc0f3c0a8 pmap (pmap) _at_ /usr/src/sys/i386/i386/pmap.c:2111
2nd 0xc0c91078 system map (system map) _at_ /usr/src/sys/vm/vm_map.c:1096
KDB: stack backtrace:
kdb_backtrace(0,ffffffff,c09c6538,c09c6600,c0976de4) at kdb_backtrace+0x29
witness_checkorder(c0c91078,9,c08ec7a3,448) at witness_checkorder+0x586
_mtx_lock_flags(c0c91078,0,c08ec7a3,448) at _mtx_lock_flags+0x6b
_vm_map_lock(c0c91000,c08ec7a3,448) at _vm_map_lock+0x23
vm_map_find(c0c91000,0,0,0,c7257a68,1000,1,7,7,4) at vm_map_find+0x2d
kmem_alloc_nofault(c0c91000,1000,c0852b67,c0a10d78,c0eacfd0) at
kmem_alloc_nofault+0x37
get_pv_entry(c0f3c0a8,0,0,bfefeffc,c7257adc) at get_pv_entry+0x110
pmap_insert_entry(c0f3c0a8,bfbff000,c0eacfd0,0,c0ca0828) at
pmap_insert_entry+0x3e
pmap_enter(c0f3c0a8,bfbff000,c0eacfd0,7,0,c0c825a0,0,c08ec1ec,380) at
pmap_enter+0x1de
vm_fault(c0f3c000,bfbff000,2,8,c0f34360) at vm_fault+0x10ac
trap_pfault(c7257c90,0,bfbfffff) at trap_pfault+0xee
trap(c0f30008,c08e0028,28,c0971eca,bfbfffff) at trap+0x3f5
calltrap() at calltrap+0x5
--- trap 0xc, eip = 0xc0855749, esp = 0xc7257cd0, ebp = 0xc7257d0c ---
subyte(0,c7257d38) at subyte+0x25
fork_exit(c0657358,0,c7257d38) at fork_exit+0xa4
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xc7257d6c, ebp = 0 ---
panic: _mtx_lock_sleep: recursed on non-recursive mutex pmap _at_
/usr/src/sys/i386/i386/pmap.c:1843

cpuid = 0
KDB: enter: panic
[thread pid 1 tid 100007 ]
Stopped at kdb_enter+0x2b: nop
db>show lockdb> show locks
exclusive sleep mutex system map r = 0 (0xc0c91078) locked _at_
/usr/src/sys/vm/vm_map.c:2313
exclusive sleep mutex pmap r = 0 (0xc0a3de20) locked _at_
/usr/src/sys/i386/i386/pmap.c:1843
exclusive sleep mutex vm page queue mutex r = 1 (0xc0a10d78) locked _at_
/usr/src/sys/i386/i386/pmap.c:1841
exclusive sx user map r = 0 (0xc0c9138c) locked _at_
/usr/src/sys/vm/vm_kern.c:466
exclusive sleep mutex Giant r = 0 (0xc09b6b68) locked _at_
/usr/src/sys/kern/vfs_mount.c:662