Subject: Run 'usermode-agent' cause kernel panic on Powerpc

From: xufeng zhang <[email-masked]>
Date: Tue, 07 Dec 2010 14:48:37 +0800

Hi Dave,

I have a question with the below patch you made before:
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powerpc/booke: Add support for advanced debug registers

From: Dave Kleikamp <[email-masked]>

Based on patches originally written by Torez Smith.
-----------------------------------------

I meet a kernel panic problem while running 'usermode-agent' on PowerPC
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Oops: Exception in kernel mode, sig: 5 [#1]
PREEMPT LTT NESTING LEVEL : 0
MPC8536 DS
last sysfs file: 
/sys/devices/f3000000.soc/f3003100.i2c/i2c-1/i2c-dev/i2c-1/dev
Modules linked in:
NIP: c00081a0 LR: c03a9560 CTR: c003547c
REGS: ef11bf10 TRAP: 2002   Not tainted  (2.6.34.6-WR4.0.0.0_standard)
MSR: 00021000 <ME,CE>  CR: 44000624  XER: 00000000
TASK = efc1de00[752] 'usermode-agent' THREAD: ef63e000
GPR00: cc00cc00 ef63fe60 efc1de00 efc1de00 efc1f700 c04c8000 00258560 
ffffffff
GPR08: ffda8a00 40000000 00001fda c0500000 49eaebbd 1008b654 3ff8a900 
00000000
GPR16: 00000000 eed84c40 c03b4570 c04ee4d0 c04ca870 ef63e03c 00000000 
00000000
GPR24: c04f7ee8 c04ee4c0 00000004 c04ca440 ef63e000 efc1f700 c04ca440 
efc1de00
NIP [c00081a0] __switch_to+0xac/0x104
LR [c03a9560] schedule+0x20c/0x3f4
Call Trace:
[ef63fe60] [efc1f700] 0xefc1f700 (unreliable)
[ef63fe70] [c03a9560] schedule+0x20c/0x3f4
[ef63fec0] [c00429e0] do_wait+0x1a4/0x278
[ef63fef0] [c0042b44] sys_wait4+0x90/0xf8
[ef63ff40] [c00106d4] ret_from_syscall+0x0/0x4
------------------------------------------

Actually, this problem is caused by enabling On Chip Debugging, when On
Chip Debugging is enabled, we enable MSR_DE as below:
#define MSR_KERNEL      (MSR_ME|MSR_RI|MSR_CE|MSR_DE)

If I comment out "mtspr(SPRN_DBCR0, thread->dbcr0);" in 
prime_debug_regs() function,
then it will be ok.

Here is my analysis for this problem:
Run 'usermode-agent' application will set Internal Debug Mode(IDM) and
Instruction Complete Debug Event(ICMP)flags for thread.
As MSR_DE is enabled, when execute context switching in prime_debug_regs(),
thread->dbcr0 would write to SPRN_DBCR0 register.
So this will enable Instruction Complete Debug Event interrupt, and it 
will cause a kernel-mode
exception right now, it will be handled in native_DebugException(), then 
kernel detected
this exception not happens in user-mode, lastly kernel call die() and 
kill current process.

So my question is could I just comment out "mtspr(SPRN_DBCR0, 
thread->dbcr0);" in prime_debug_regs()?
I'm sure whether or not it will impose a bad impact on debugging.
Thanks in advance!

Thanks,
Xufeng Zhang