Re: [Xenomai-help] Page fault in real time task causes lockup

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Gilles Chanteperdrix wrote:
> Steve Deiters wrote:
>>> -----Original Message-----
>>> From: xenomai-help-bounces@domain.hid 
>>> [mailto:xenomai-help-bounces@domain.hid] On Behalf Of Steve Deiters
>>> Sent: Friday, August 13, 2010 5:15 PM
>>> To: xenomai@xenomai.org
>>> Subject: [Xenomai-help] Page fault in real time task causes lockup
>>>
>>> I'm trying to track down a problem where it seems that a page 
>>> fault is causing a lockup on my machine.  I am running on a 
>>> PowerPC with Linux version 2.6.33.5 and Xenomai 2.5.4, but 
>>> also saw the same thing with Xenomai 2.5.3.
>>>
>>> What I am doing is mmaping a FPGA on the parallel bus in my 
>>> task initialization.  Later on I have a interrupt loop which 
>>> uses rt_intr_wait to service some FPGA stuff.  On access to 
>>> some of my FPGA mapped registers I get a page fault which 
>>> causes a lockup.  I'm guessing there is some interaction 
>>> going on with the rt_intr_wait and the fault exception.  If I 
>>> prefault the map by reading some of the registers before the 
>>> loop it is ok.  If I change the rt_intr_wait to a timed loop 
>>> using rt_wait_period and don't prefault the registers it is ok.
>>>
>>> If I enable T_WARNSW I get a SIGXCPU when it tries to access 
>>> the mapped registers.  I don't necessarily care that it 
>>> faults there so I don't want to have to prefault like I am doing.
>>>
>>> If I enable some of the debugging options I end up with the 
>>> following exception dump:
>>>
>>> -----------
>>>
>>> [   23.623184] Xenomai: Switching  to secondary mode after exception
>>> #769 from user-space at 0xff187ac (pid 586)
>>> [   23.634273] Xenomai: Switching  to secondary mode after exception
>>> #769 from user-space at 0xff187ac (pid 587)
>>> [   23.653414] Xenomai: Switching  to secondary mode after exception
>>> #769 from user-space at 0xff187ac (pid 592)
>>> [   23.675243] Xenomai: Switching dsp_task to secondary mode after
>>> exception #769 from user-space at 0x10016634 (pid 595)
>>> [   24.456360] Xenomai: Switching dsp_task to secondary mode after
>>> exception #769 from user-space at 0x10002d28 (pid 595)
>>> [   24.467285] I-pipe: Detected illicit call from domain 'Xenomai'
>>> [   24.467300] <3>        into a service reserved for domain 
>>> 'Linux' and
>>> below.
>>> [   24.480199] Xenomai: Switching dsp_task to secondary mode after
>>> exception #1792 in kernel-space at 0xc0062f48 (pid 595)
>>> [   24.491109] Oops: Exception in kernel mode, sig: 5 [#1]
>>> [   24.496258] PREEMPT MPC5121 BE
>>> [   24.499300] Modules linked in: lpcmem axe immmem
>>> [   24.503912] NIP: c0062f48 LR: c0025b0c CTR: c01be5b0
>>> [   24.508870] REGS: c7bc3c60 TRAP: 0700   Not tainted  (2.6.33.5)
>>> [   24.514775] MSR: 00021032 <ME,CE,IR,DR>  CR: 24000422  
>>> XER: 20000000
>>> [   24.521127] TASK = c7b30550[595] 'dsp_task' THREAD: c7bc2000
>>> [   24.526600] GPR00: 00000001 c7bc3d10 c7b30550 c03ac1c0 00002a39
>>> ffffffff c0360000 c03ac1c0
>>> [   24.534946] GPR08: 00000000 000028ff 00002900 c0360000 82000442
>>> 1003c7b8 00000001 c0360000
>>> [   24.543292] GPR16: c03b0000 c7bc3f50 00008000 c0300000 c03b0000
>>> c0360000 00000003 c0360000
>>> [   24.551638] GPR24: c0360000 c7bc3d3c 0000009c c7bc2000 0000000f
>>> c7bc3d4b c039d918 00000001
>>> [   24.560180] NIP [c0062f48] __ipipe_unstall_root+0x34/0x80
>>> [   24.565564] LR [c0025b0c] vprintk+0x340/0x444
>>> [   24.569895] Call Trace:
>>> [   24.572336] [c7bc3d10] [c7bc3d4b] 0xc7bc3d4b (unreliable)
>>> [   24.577729] [c7bc3d20] [c0025b0c] vprintk+0x340/0x444
>>> [   24.582770] [c7bc3db0] [c0026304] printk+0xb8/0x1f8
>>> [   24.587640] [c7bc3e00] [c006256c] ipipe_check_context+0xc4/0xcc
>>> [   24.593555] [c7bc3e10] [c0299538] __down_interruptible+0xb4/0x148
>>> [   24.599643] [c7bc3e40] [c004799c] down_interruptible+0xcc/0xdc
>>> [   24.605470] [c7bc3e60] [c0075acc] xnshadow_harden+0x64/0x248
>>> [   24.611114] [c7bc3e80] [c0075d4c] losyscall_event+0x9c/0x374
>>> [   24.616766] [c7bc3ed0] [c0063bc0] __ipipe_dispatch_event+0x98/0x1f0
>>> [   24.623025] [c7bc3f20] [c000bcf0] __ipipe_syscall_root+0x60/0x170
>>> [   24.629108] [c7bc3f40] [c00133e4] DoSyscall+0x20/0x5c
>>> [   24.634151] --- Exception: c01 at 0xff19c94
>>> [   24.634158]     LR = 0xff19c08
>>> [   24.641360] Instruction dump:
>>> [   24.644318] 7c0802a6 90010014 7c0000a6 5400045e 7c000124 3d60c036
>>> 3d20c03b 814b2858
>>> [   24.652055] 3929c1c0 7d4a4a78 312affff 7c095110 <0f000000> 3d60c036
>>> 38600000 392b14f8
>>> [   24.660058] ------------[ cut here ]------------
>>> [   24.664600] kernel BUG at kernel/ipipe/core.c:311!
>>> [   24.669413] ---[ end trace ca02c1a54b14d664 ]---
>>> [   24.674021] note: dsp_task[595] exited with preempt_count 1
>>>
>>
>> If this gives any more clues, if I comment out the section in
>> __rt_intr_wait in native/syscall.c where it raises the priority to
>> XNSCHED_IRQ_PRIO it does not lock up. 
> 
> This is strange, it looks like the thread wants to move from secondary
> mode to primary mode while it is already running in primary mode.
> 
The most probable reason being that the previous call to xnshadow_relax
went in fact wrong. The thing that could go wrong would be
xnpod_suspend_thread in xnshadow_relax not suspending the thread.

-- 
					    Gilles.