strace causes panic: sleeping thread

I see this on freshly build CURRENT (i386):

Using strace causes an immediate panic, e.g. strace echo foo:

[GDB will not be able to debug user-mode threads: /usr/lib/libthread_db.so: 
Undefined symbol "ps_pglobal_lookup"]
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Unread portion of the kernel message buffer:
Sleeping thread (tid 100078, pid 896) owns a non-sleepable lock
sched_switch(c2da4a20,0,1) at sched_switch+0xff
mi_switch(1,0) at mi_switch+0x1d4
sleepq_switch(c2f7e168) at sleepq_switch+0x8b
sleepq_wait_sig(c2f7e168,c2da4a20,5c,0,100,...) at sleepq_wait_sig+0x1d
_sleep(c2f7e168,c2f7e060,15c,c0759778,0,...) at _sleep+0x26e
procfs_ioctl(c2da4a20,c2f7e000,c2b27200,40147004,c2ca3b80) at 
procfs_ioctl+0x1f5
pfs_ioctl(d4e18b84) at pfs_ioctl+0x60
VOP_IOCTL_APV(c0794f60,d4e18b84) at VOP_IOCTL_APV+0x38
vn_ioctl(c2ba2480,40147004,c2ca3b80,c2f60180,c2da4a20) at vn_ioctl+0x18d
kern_ioctl(c2da4a20,3,40147004,c2ca3b80) at kern_ioctl+0x282
ioctl(c2da4a20,d4e18d00) at ioctl+0xf1
syscall(d4e18d38) at syscall+0x2a2
Xint0x80_syscall() at Xint0x80_syscall+0x20
--- syscall (54, FreeBSD ELF32, ioctl), eip = 0x2816474f, esp = 0xbfbfe38c, 
ebp = 0xbfbfe418 ---
panic: sleeping thread
cpuid = 0
KDB: enter: panic
Uptime: 21s
Physical memory: 470 MB
Dumping 42 MB: 27 11

#0  doadump () at pcpu.h:172
172	pcpu.h: No such file or directory.
	in pcpu.h
(kgdb) bt
#0  doadump () at pcpu.h:172
#1  0xc055bfe1 in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:409
#2  0xc055c323 in panic (fmt=0xc07619aa "sleeping thread")
    at /usr/src/sys/kern/kern_shutdown.c:563
#3  0xc0584384 in propagate_priority (td=0xc2da4a20)
    at /usr/src/sys/kern/subr_turnstile.c:205
#4  0xc0584d90 in turnstile_wait (lock=0xc2f7e060, owner=0xc2da4a20, queue=0)
    at /usr/src/sys/kern/subr_turnstile.c:682
#5  0xc0552d87 in _mtx_lock_sleep (m=0xc2f7e060, tid=3269086160, opts=0, 
    file=0x0, line=0) at /usr/src/sys/kern/kern_mutex.c:415
#6  0xc0567658 in thread_suspend_check (return_instead=0)
    at /usr/src/sys/kern/kern_thread.c:830
#7  0xc0583d57 in userret (td=0xc2da4bd0, frame=0xd4e1bd38)
    at /usr/src/sys/kern/subr_trap.c:112
#8  0xc070d0c5 in syscall (frame=0xd4e1bd38)
    at /usr/src/sys/i386/i386/trap.c:1078
#9  0xc06f60c0 in Xint0x80_syscall () 
at /usr/src/sys/i386/i386/exception.s:196
#10 0x00000033 in ?? ()
Previous frame inner to this frame (corrupt stack?)
(kgdb)