Panic on -CURRENT after LDT changes

On a -CURRENT build today with Jeff's LDT locking changes, I get an
instant, reproducible panic by doing:

# cd /usr/src
# make -DNO_CLEAN buildworld

Backing out the LDT changes to /sys/i386/i386/*machdep* corrects the
problem.  Here is a dmesg and backtrace.  In frame 6, it appears mdp is
NULL (i.e. td->td_proc->p_md = 0x0).

http://www.marcuscom.com/downloads/JCLARKE-PC_dump.txt

Unread portion of the kernel message buffer:
kernel trap 12 with interrupts disabled


Fatal trap 12: page fault while in kernel mode
fault virtual address   = 0x4
fault code              = supervisor read, page not present
instruction pointer     = 0x20:0xc0723195
stack pointer           = 0x28:0xe768dbf4
frame pointer           = 0x28:0xe768dc20
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = resume, IOPL = 0
current process         = 1025 (make)
trap number             = 12
panic: page fault
Uptime: 2m37s
Physical memory: 2022 MB
Dumping 65 MB: 50 34 18 2

#0  doadump () at pcpu.h:172
#1  0xc0562c93 in boot (howto=260)
at /usr/src/sys/kern/kern_shutdown.c:409
#2  0xc0562f18 in panic (fmt=Variable "fmt" is not available.
#3  0xc07240b2 in trap_fatal (frame=0xe768dbb4, eva=4)
    at /usr/src/sys/i386/i386/trap.c:876
#4  0xc07249a3 in trap (frame=0xe768dbb4)
at /usr/src/sys/i386/i386/trap.c:276
#5  0xc070de5b in calltrap () at /usr/src/sys/i386/i386/exception.s:139
#6  0xc0723195 in i386_ldt_grow (td=Variable "td" is not available.
)
    at /usr/src/sys/i386/i386/sys_machdep.c:704
#7  0xc072350a in i386_set_ldt (td=0xc4fccbd0, uap=0xe768dc68,
    descs=0xe76f3000) at /usr/src/sys/i386/i386/sys_machdep.c:631
#8  0xc0723a44 in sysarch (td=0xc4fccbd0, uap=0xe768dcfc)
    at /usr/src/sys/i386/i386/sys_machdep.c:134
#9  0xc0724645 in syscall (frame=0xe768dd38)
    at /usr/src/sys/i386/i386/trap.c:1016
#10 0xc070dec0 in Xint0x80_syscall ()
at /usr/src/sys/i386/i386/exception.s:196
#11 0x00000033 in ?? ()

Joe

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