panic: lock (sleep mutex) vm object not locked _at_ vm_map.c:1433

With GENERIC HEAD from Dec 20 12:18 UTC I got:

panic(c082f17c,c08448df,c0845279,c084494b,599) at panic+0xd8
witness_unlock(c105d8c4,8,c0844942,599) at witness_unlock+0x170
_mtx_unlock_flags(c105d8c4,0,c0844942,599,0,0,1d000,0) at
_mtx_unlock_flags+0x3b
vm_map_insert(c2072960,c105d8c4,0,0,28060000,1d000,5,7,0,c2072960,c0827d62,156)
at vm_map_insert+0x1ad
elf32_map_insert(0,0,28060000,2807d000,5,7,10a) at
elf32_map_insert+0x21e
elf32_load_section(c1787114,c105d8c4,0,28060000,1c96a,1c96a,5,1000)
at elf32_load_section+0x202
elf32_load_file(cf68faa8,cf68fbd0,1000) at elf32_load_file+0x3a6
exec_elf32_imgact(cf68fb94) at exec_elf32_imgact+0x413
kern_execve(c35a68a0,8068384,80683a4,8068404,0) at
kern_execve+0x3fa
execve(c35a68a0,cf68fd14,3,0,286) at execve+0x18
syscall(2f,2f,2f,0,8068384) at syscall+0x128
Xint0x80_syscall() at Xint0x80_syscall+0x1f
--- syscall (59, FreeBSD ELF32, execve), eip = 0x28127767, esp =
0xbfbfe7fc, ebp = 0xbfbfe828 ---

Details at http://www.holm.cc/stress/log/cons97.html
-- 
Peter Holm