panic: _mtx_lock_sleep: recursed on non-recursive mutex system map @ ../../../vm/vm_map.c:2997

I have got this panic 3 times tonight on a 12-processor sparc64 e4500,
but only once did it get as far as displaying a stack trace before
hanging.  WITNESS was enabled (without WITNESS_SKIPSPIN) but did not
report anything.

panic: _mtx_lock_sleep: recursed on non-recursive mutex system map _at_ ../../../vm/vm_map.c:2997

cpuid = 3
KDB: enter: panic
KDB: stack backtrace:
paiic:_lrocjm atoicl
croc+ x 3
foBk_sxatk)bac foak_:
_fink(tr   ol  e ) a  f    tr       e 0x
                                          at sched_bind+0x78
boot() at boot+0x24
panic() at panic+0x1bc
longjmp() at longjmp+0x44
trap() at trap+0x220
-- fast data access mmu miss tar=0xe9f72000 %o7=0xc02f8284 --
cpu_ipi_selected() at cpu_ipi_selected+0x2c
ipi_selected() at ipi_selected+0x14
stop_cpus() at stop_cpus+0x1c
kdb_trap() at kdb_trap+0x64
trap() at trap+0x264
-- breakpoint %o7=0xc0191f74 --
kdb_enter() at kdb_enter+0x3c
panic() at panic+0x164
_mtx_lock_sleep() at _mtx_lock_sleep+0x40
_mtx_lock_flags() at _mtx_lock_flags+0xa4
_vm_map_lock_read() at _vm_map_lock_read+0x1c
vm_map_lookup() at vm_map_lookup+0x1c
vm_fault() at vm_fault+0x68
trap_pfault() at trap_pfault+0x1a8
trap() at trap+0x28c
-- fast data access mmu miss tar=0xe9f9a000 %o7=0xc02cea68 --
vm_map_entry_splay() at vm_map_entry_splay+0x10
vm_map_find() at vm_map_find+0x34
kmem_alloc_nofault() at kmem_alloc_nofault+0x44
pmap_pinit() at pmap_pinit+0x30
vmspace_zinit() at vmspace_zinit+0x14
slab_zalloc() at slab_zalloc+0x264
uma_zone_slab() at uma_zone_slab+0x12c
uma_zalloc_bucket() at uma_zalloc_bucket+0x16c
uma_zalloc_arg() at uma_zalloc_arg+0x374
vmspace_alloc() at vmspace_alloc+0x14
vmspace_fork() at vmspace_fork+0x24
vm_forkproc() at vm_forkproc+0xe4
fork1() at fork1+0xd44
fork() at fork+0x10
syscall() at syscall+0x2dc
-- syscall (2, FreeBSD ELF64, fork) %o7=0x110020 --
-userland() at 0x40635568
user trace: trap %o7=0x110020
pc 0x40635568, sp 0x7fdffffd721
pc 0x106890, sp 0x7fdffffd7e1
pc 0x105fe4, sp 0x7fdffffd9a1
pc 0x107428, sp 0x7fdffffdae1
pc 0x110b88, sp 0x7fdffffdbc1
pc 0x102354, sp 0x7fdffffdce1
pc 0x40227474, sp 0x7fdffffdda1
done
panic: mi_switch: did not reenter debugger
cpuid = 3
KDB: stack backtrace:

[At this point it looped recursively panicking for 5 minutes or so before hanging].

Kris