From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tom Cook <tom.k.cook@gmail.com>
Subject: Tracers+cyclictest causing kernel oops
Date: Fri, 26 Apr 2013 09:04:48 +0100
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I'm running 3.6.11-rt31 on a Raspberry Pi.  Running cyclictest like this:

  sudo ./cyclictest -p95 -m -W -b 1000
  # /dev/cpu_dma_latency set to 0us
  policy: fifo: loadavg: 0.72 0.28 0.10 2/137 2054

causes this:

Entering kdb (current=0xca78e0a0, pid 2058) Oops: (null)
due to oops @ 0xc009f654

Pid: 2058, comm:           cyclictest
CPU: 0    Not tainted  (3.6.11-rt31+ #10)
PC is at ring_buffer_lock_reserve+0x7c/0x144
LR is at ftrace_test_stop_func+0x28/0x34
pc : [<c009f654>]    lr : [<c0097684>]    psr: 600001d1
sp : ca7bfa78  ip : ca7bfaf0  fp : ca7bfaec
r10: 00000000  r9 : c088ece0  r8 : ca7be000
r7 : 200001d1  r6 : c030dd28  r5 : c00ab928  r4 : c0786000
r3 : 200001d1  r2 : 00000018  r1 : 00000000  r0 : cb8013a0
Flags: nZCv  IRQs off  FIQs off  Mode FIQ_32  ISA ARM  Segment user
Control: 00c5387d  Table: 0a7b8008  DAC: 00000015
[<c0015188>] (unwind_backtrace+0x0/0xfc) from [<c041953c>]
(dump_stack+0x20/0x24)
[<c041953c>] (dump_stack+0x20/0x24) from [<c000f720>] (show_regs+0x4c/0x58)
[<c000f720>] (show_regs+0x4c/0x58) from [<c0086c48>] (kdb_dumpregs+0x38/0x60)
[<c0086c48>] (kdb_dumpregs+0x38/0x60) from [<c00897d8>]
(kdb_main_loop+0x348/0x77c)
[<c00897d8>] (kdb_main_loop+0x348/0x77c) from [<c008c3d8>]
(kdb_stub+0x2f0/0x3f8)
[<c008c3d8>] (kdb_stub+0x2f0/0x3f8) from [<c0082b24>]
(kgdb_handle_exception+0x164/0x6fc)
[<c0082b24>] (kgdb_handle_exception+0x164/0x6fc) from [<c00147e0>]
(kgdb_notify+0x3c/0x74)
[<c00147e0>] (kgdb_notify+0x3c/0x74) from [<c0421e84>]
(notifier_call_chain+0x54/0x94)
[<c0421e84>] (notifier_call_chain+0x54/0x94) from [<c0421f0c>]
(__atomic_notifier_call_chain+0x48/0x5c)
[<c0421f0c>] (__atomic_notifier_call_chain+0x48/0x5c) from
[<c0421f48>] (atomic_notifier_call_chain+0x28/0x30)
[<c0421f48>] (atomic_notifier_call_chain+0x28/0x30) from [<c0421f90>]
(notify_die+0x40/0x4c)
[<c0421f90>] (notify_die+0x40/0x4c) from [<c0011de8>] (die+0xd8/0x3c0)
[<c0011de8>] (die+0xd8/0x3c0) from [<c0012480>] (bad_mode+0x4c/0x70)
[<c0012480>] (bad_mode+0x4c/0x70) from [<c0097684>]
(ftrace_test_stop_func+0x28/0x34)

AFAICT, this is actually kgdb encountering a problem while trying to
unwind the stack trace from the real problem - is that right?

The problem seems to be caused by cyclictest setting up the tracer,
not triggering it - it doesn't matter what I set the latency threshold
to, the same thing happens.

Is there a known cause for this?  Any tips on how to start debugging it?

Thanks,
Tom