kgdb strange behaviour on current

[Victor Snezhko]

  Some days ago I analysed the crashdump of a panic triggered by ipv6
changes on 2005.10.21.16.23.xx, and noticed a strange thing. See:

/home/vvs # kgdb /usr/obj/usr/src/sys/VVS/kernel.debug /var/crash/vmcore.1
[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:
kernel trap 12 with interrupts disabled


Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address	= 0xdeadc0e6
fault code		= supervisor read, page not present
instruction pointer	= 0x20:0xc066e0c2
stack pointer	        = 0x28:0xcbfd3cac
frame pointer	        = 0x28:0xcbfd3cd4
code segment		= base 0x0, limit 0xfffff, type 0x1b
			= DPL 0, pres 1, def32 1, gran 1
processor eflags	= resume, IOPL = 0
current process		= 12 (swi4: clock sio)
panic: from debugger
cpuid = 0
Uptime: 1m26s
Dumping 255 MB (2 chunks)
  chunk 0: 1MB (159 pages) ... ok
  chunk 1: 255MB (65264 pages) 239 223 207 191 175 159 143 127 111 95 79 63 47 31 15

#0  doadump () at pcpu.h:165
165	pcpu.h: No such file or directory.
	in pcpu.h

(kgdb) bt
#0  doadump () at pcpu.h:165
#1  0xc06613fc in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:399
#2  0xc0661711 in panic (fmt=0xc0859480 "from debugger")
    at /usr/src/sys/kern/kern_shutdown.c:555
#3  0xc046d151 in db_panic (addr=-1066999614, have_addr=0, count=-1, 
    modif=0xcbfd3aa8 "") at /usr/src/sys/ddb/db_command.c:435
#4  0xc046d0e8 in db_command (last_cmdp=0xc094a164, cmd_table=0x0, 
    aux_cmd_tablep=0xc08c0048, aux_cmd_tablep_end=0xc08c0064)
    at /usr/src/sys/ddb/db_command.c:404
#5  0xc046d1b0 in db_command_loop () at /usr/src/sys/ddb/db_command.c:455
---Type <return> to continue, or q <return> to quit---
#6  0xc046edc9 in db_trap (type=12, code=0) at /usr/src/sys/ddb/db_main.c:221
#7  0xc067b408 in kdb_trap (type=12, code=0, tf=0xcbfd3c6c)
    at /usr/src/sys/kern/subr_kdb.c:485
#8  0xc0823f68 in trap_fatal (frame=0xcbfd3c6c, eva=3735929062)
    at /usr/src/sys/i386/i386/trap.c:846
#9  0xc08235f2 in trap (frame=
      {tf_fs = 8, tf_es = -872611800, tf_ds = -1067122648, tf_edi = -1049930560, tf_esi = -1051228800, tf_ebp = -872596268, tf_isp = -872596328, tf_ebx = 1, tf_edx = -966778392, tf_ecx = -559038242, tf_eax = 85115, tf_trapno = 12, tf_err = 0, tf_eip = -1066999614, tf_cs = 32, tf_eflags = 589826, tf_esp = -1051359088, tf_ss = 0})
---Type <return> to continue, or q <return> to quit---
    at /usr/src/sys/i386/i386/trap.c:269
#10 0xc0810e2a in calltrap () at /usr/src/sys/i386/i386/exception.s:139
#11 0xc066e0c2 in softclock (dummy=0x0) at /usr/src/sys/kern/kern_timeout.c:220
#12 0xc064ed32 in ithread_execute_handlers (p=0xc1558890, ie=0xc1578580)
    at /usr/src/sys/kern/kern_intr.c:662
#13 0xc064ee63 in ithread_loop (arg=0xc151f8e0)
    at /usr/src/sys/kern/kern_intr.c:745
#14 0xc064dfc4 in fork_exit (callout=0xc064edfc <ithread_loop>, arg=0xc151f8e0, 
    frame=0xcbfd3d38) at /usr/src/sys/kern/kern_fork.c:789
#15 0xc0810e8c in fork_trampoline () at /usr/src/sys/i386/i386/exception.s:208
(kgdb) up 11
#11 0xc066e0c2 in softclock (dummy=0x0) at /usr/src/sys/kern/kern_timeout.c:220
220				if (c->c_time != curticks) {
(kgdb) list
215			curticks = softticks;
216			bucket = &callwheel[curticks & callwheelmask];
217			c = TAILQ_FIRST(bucket);
218			while (c) {
219				depth++;
220				if (c->c_time != curticks) {
221					c = TAILQ_NEXT(c, c_links.tqe);
222					++steps;
223					if (steps >= MAX_SOFTCLOCK_STEPS) {
224						nextsoftcheck = c;
(kgdb) print c
$1 = (struct callout *) 0xdeadc0de
(kgdb) print bucket->tqh_first
$2 = (struct callout *) 0xc18eb520
(kgdb) print *(bucket->tqh_first)
$3 = {c_links = {sle = {sle_next = 0xdeadc0de}, tqe = {tqe_next = 0xdeadc0de, 
      tqe_prev = 0xdeadc0de}}, c_time = -559038242, c_arg = 0xdeadc0de, 
  c_func = 0xdeadc0de, c_mtx = 0xdeadc0de, c_flags = -559038242}
(kgdb) print bucket
$4 = (struct callout_tailq *) 0xc66021e8
(kgdb) quit

/home/vvs # grep "#define.*[^S]TAILQ_FIRST" /usr/src/sys/sys/queue.h
#define	TAILQ_FIRST(head)	((head)->tqh_first)

bucket->tqh_first seems to be a valid pointer, but points to
garbage. But c should be the same pointer, and debugger shows a
distinct value for it!  

Do we need to debug this further? What can I do?

-- 
WBR, Victor V. Snezhko
EMail: snezhko at indorsoft.ru