looking for help interpreting softlockup/stack trace

["Chris Friesen" <cfriesen@nortel.com>]

I've run into an interesting issue with an Octeon-based board, where it 
just seems to hang.  I suspect we're hitting some kind of locking bug, 
and I'm trying to track it down.  If it matters, the kernel is quite old 
(heavily patched 2.6.14) and I've got no chance of upgrading it.  (The 
usual embedded scenario.)

I've added some scheduler instrumentation, as well as adding a stack 
dump to the output of the softlockup code.

In the trace below, is "epc" the program counter at the time of the 
timer interrupt?  How does "ra" fit into this, given that the function 
whose address it contains isn't seen in the stack trace until quite a 
ways down?

Any insights are greatly appreciated...

Chris

cpu9: jiffies: 4295366417, hrtime: 552015654907, 500 ms without calling 
schedule() since scheduler requested
cpu1: jiffies: 4295366418, hrtime: 552016450325, 500 ms without calling 
schedule() since scheduler requested
cpu3: jiffies: 4295366442, hrtime: 552034584838, 500 ms without calling 
schedule() since scheduler requested
cpu9: jiffies: 4295367258, hrtime: 552646827014, 841 ms between 
scheduler_tick() calls
BUG: soft lockup detected on CPU#0!
Cpu 0
$ 0   : 0000000000000000 0000000000000000 ffffffff80000000 ffffffff00000000
$ 4   : a8000000db3ce808 0000000000000084 a8000000d34cb1ea a8000000d34cb1ea
$ 8   : 0000000045000084 0000000000000002 0000000000000001 ffffffff8165e1e0
$12   : 0000000000000000 ffffffff812dedac 0000500000008000 0000100000000000
$16   : a8000000d3a40b68 a8000000db3ce580 a8000000d3a40b00 ffffffff8151e448
$20   : a8000000db3ce808 ffffffff81593808 a8000000d3a40b00 0000000000000000
$24   : 0000000000000030 c000000001ae1820
$28   : ffffffff81590000 ffffffff81593680 ffffffff81593810 c000000001b4ab9c
Hi    : 0000000000000000
Lo    : 0000000000000000
epc   : ffffffff8151e44c _read_lock+0x4/0x20     Tainted: P
ra    : c000000001b4ab9c $LBB378+0x0/0x18 [vnb]
Status: 1000ffe3    KX SX UX KERNEL EXL IE
Cause : 40808000
PrId  : 000d0301
Call Trace:
  [<ffffffff811a251c>] softlockup_tick+0x12c/0x158
  [<ffffffff811126dc>] timer_interrupt+0xd4/0x4c8
  [<ffffffff81112890>] timer_interrupt+0x288/0x4c8
  [<ffffffff81101d50>] octeon_main_timer_interrupt+0x78/0x90
  [<ffffffff811a2c24>] handle_IRQ_event+0x45c/0x1528
  [<ffffffff811a3dc4>] __do_IRQ+0xd4/0x230
  [<ffffffff8151e448>] _read_lock+0x0/0x20
  [<ffffffff8110c4a8>] do_IRQ+0x440/0x1520
  [<ffffffff8139263c>] neigh_lookup+0xb4/0x128
  [<ffffffff8151e448>] _read_lock+0x0/0x20
  [<ffffffff8110a430>] ret_from_irq+0x0/0x10
  [<ffffffff8114fbdc>] __wake_up_common+0x6c/0xb8
  [<ffffffff812dedac>] memset_partial+0x48/0x60
  [<ffffffff8151e448>] _read_lock+0x0/0x20
  [<c000000001ae1820>] $Ltext0+0x0/0x4 [vnb]
  [<ffffffff8151e618>] _read_unlock+0x0/0x20
  [<c000000001b4ab9c>] $LBB378+0x0/0x18 [vnb]
  [<ffffffff8151e44c>] _read_lock+0x4/0x20
  [<ffffffff81407fcc>] arp_rcv+0x18c/0x258
  [<ffffffff8138b84c>] netif_dispatch_skb+0x18c/0x2d0
  [<c000000000861208>] $LBE370+0x0/0x10 [cavium_ethernet]
  [<c000000001ae2acc>] $L396+0x0/0x1c [vnb]
  [<c000000001b47ba8>] $LCFI35+0x8c/0x9c [vnb]
  [<c000000001adb36c>] $L1193+0x2c/0x48 [vnb]
  [<c000000001b3dc5c>] $L1047+0x28/0xd0 [vnb]
  [<c000000001b3d6f0>] $LBB344+0x2c/0x34 [vnb]
  [<ffffffff811476a8>] activate_task+0x98/0x108
  [<c000000001adb4cc>] $L1215+0x2c/0x30 [vnb]
  [<c000000001adb7c0>] $L1273+0x0/0x18 [vnb]
  [<c000000001ae18a8>] $L28+0x24/0x4c [vnb]
  [<ffffffff8138bc90>] netif_receive_skb+0x300/0x730
  [<ffffffff8138130c>] __alloc_skb+0x84/0x1a8
  [<c000000000861970>] $L620+0x20/0x28 [cavium_ethernet]
  [<ffffffff811734e8>] process_timeout+0x0/0xb08
  [<ffffffff81111cc0>] do_gettimeofday+0xa8/0x220
  [<ffffffff81169b2c>] tasklet_action+0x684/0x1950
  [<ffffffff81187b9c>] hrtimer_run_queues+0x84/0x1a0
  [<c0000000008613d8>] cvm_oct_tasklet_rx+0x0/0x4 [cavium_ethernet]
  [<ffffffff81171ce4>] run_timer_softirq+0x20c/0xce0
  [<c0000000008613d8>] cvm_oct_tasklet_rx+0x0/0x4 [cavium_ethernet]
  [<ffffffff81112890>] timer_interrupt+0x288/0x4c8
  [<ffffffff81167afc>] __do_softirq+0x56c/0x1818
  [<ffffffff81168e40>] do_softirq+0x98/0xb8
  [<ffffffff8110c874>] do_IRQ+0x80c/0x1520
  [<ffffffff8110dd98>] kernel_thread+0xa0/0xbf8
  [<ffffffff8110a430>] ret_from_irq+0x0/0x10
  [<ffffffff8151bef4>] schedule+0xd4c/0x1a98
  [<ffffffff81109ec0>] r4k_wait+0x0/0x10
  [<ffffffff813859b0>] datagram_poll+0x0/0x140
  [<ffffffff8110d974>] cpu_idle+0x44/0x60
  [<ffffffff81109ec8>] r4k_wait+0x8/0x10
  [<ffffffff81611e70>] start_kernel+0x460/0x4f8
  [<ffffffff81611e68>] start_kernel+0x458/0x4f8
  [<ffffffff81611000>] kernel_entry+0x0/0x4c