Fatal trap 12 in kern/kern_kse.c:998

Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address   = 0x10
fault code              = supervisor write, page not present
instruction pointer     = 0x8:0xc064fdd0
stack pointer           = 0x10:0xd1818c84
frame pointer           = 0x10:0xd1818ca4
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = resume, IOPL = 0
current process         = 1399 (pthread)
[thread 100689]
Stopped at      thread_update_usr_ticks+0x108:  orl     $0x1,0x10(%eax)

(kgdb) l *0xc064fdd0
0xc064fdd0 is in thread_update_usr_ticks (../../../kern/kern_kse.c:998).
993                     }
994             }
995             kg = td->td_ksegrp;
996             if (kg->kg_upquantum && ticks >= kg->kg_nextupcall) {
997                     mtx_lock_spin(&sched_lock);
998                     td->td_upcall->ku_flags |= KUF_DOUPCALL;
999                     mtx_unlock_spin(&sched_lock);
1000            }
1001            return (0);
1002    }

I've included a patch suggestion, that seems to do the trick for me.
-- 
Peter Holm