panic: sleeping thread owns a mutex

One of the alpha package machines just died with the following:

panic: sleeping thread owns a mutex
panic() at panic+0x160
propagate_priority() at propagate_priority+0x148
_mtx_lock_sleep() at _mtx_lock_sleep+0x264
_mtx_lock_flags() at _mtx_lock_flags+0x84
_vm_map_lock() at _vm_map_lock+0x40
vm_map_remove() at vm_map_remove+0x34
kmem_free() at kmem_free+0x34
pipe_free_kmem() at pipe_free_kmem+0xbc
pipeclose() at pipeclose+0x188
pipe_close() at pipe_close+0x40
fdrop_locked() at fdrop_locked+0x180
fdrop() at fdrop+0x50
closef() at closef+0x260
fdfree() at fdfree+0x3c4
exit1() at exit1+0x578
sys_exit() at sys_exit+0x58
syscall() at syscall+0x338
XentSys() at XentSys+0x64
--- syscall (1, FreeBSD ELF64, sys_exit) ---
--- user mode ---
db>

(gdb -k is still broken on alpha, so I can't do better).  The machine
is running a kernel from June 20.

Kris