Fatal trap 12 in kern_ptrace

Recently I've been seeing libpthreaded processes just hang suddenly. ps
shows them in the STOPped state, and ps -H shows that all threads are
idle except for one, which is waiting on a lock (I've seen ufs and wait
on different occasions).  Killing the process results in an instant
system freeze.  I tried gdb'ing one this time, and was rewarded with
the following trap:

 kernel trap 12 with interrupts disabled


 Fatal trap 12: page fault while in kernel mode
 cpuid = 0; apic id = 03
 fault virtual address   = 0x44
 fault code              = supervisor write, page not present
 instruction pointer     = 0x8:0xc058ae4c
 stack pointer           = 0x10:0xdef5db84
 frame pointer           = 0x10:0xdef5dc04
 code segment            = base 0x0, limit 0xfffff, type 0x1b
                         = DPL 0, pres 1, def32 1, gran 1
 processor eflags        = resume, IOPL = 0
 current process         = 63683 (gdb)

At this point the system froze so I couldn't get a full stack trace,
but:

$ addr2line -f -e kernel.debug 0xc058ae4c
kern_ptrace
../../../kern/sys_process.c:659

658              if (saved_pid <= PID_MAX) {
659                      p->p_xthread->td_flags &= ~TDF_XSIG;
660                      p->p_xthread->td_xsig = data;

It looks like p_xthread is null when it shouldn't be.  This is an Aug 02
kernel, so no PREEMPTION.

-- 
	Dan Nelson
	dnelson_at_allantgroup.com