Re: [Xenomai] BUG: Unhandled exception over domain Xenomai - switching to ROOT

[Philippe Gerum <rpm@xenomai.org>]

On 07/12/2012 11:29 PM, Jorge Ramirez Ortiz, HCL Europe wrote:
> Hi Gilles
>
> Information and context below.
>
> kernel: 2.6.35.9
> xenomai:2.6.0
> ipipe:2.8-04
> cpu atom z530 1.6HH
>
> Error
>>>> [  242.962195] BUG: Unhandled exception over domain Xenomai at 0x892160 - switching to ROOT
>>>> [  242.990052] Pid: 972, comm: InterruptTest Not tainted 2.6.35.9-prot-xeno-atom #4
>>>> [  243.015979] Call Trace:
>>>> [  243.025121]  [<c041b0b3>] __ipipe_handle_exception+0x203/0x210
>>>> [  243.048691]  [<c07ef4fb>] error_code+0x63/0x70
>
> System:
> 1. Xenomai thread calling ~9000 PCI writes in a loop. Each of these 9000 IOCTLs is processed in the realtime path of the RTDM driver.
> 2. A library has an interrupt notifier on the same RTDM device.
>     The interrupt notifier is a wait queue in the non-realtime path of the RTDM driver. Events are raised from the RTDM interrupt handler to notify the sleeping threads.
>     The interrupt notifier is a timed wait every 100ms. When it times out, the linux application reads (msgrcv) from a queue in a shared memory segment (shmget created).
>
> The problem with the design is obvious and the issue above solved by modifying the library.
> The interrupt notifier should be sleeping in an rtdm_event_t in the realtime path of the device instead of a linux wait queue.
>
> However, the other design, even if wrong, shouldnt have caused exceptions.

Your assumption is wrong: you cannot parry a real-time context 
reentering the regular linux kernel from an unsafe place through a plain 
function call, absolutely none. We can only detect this situation after 
the facts to give some debug hints, hopefully before the complete crash, 
by instrumenting code paths which may be spuriously called that way, e.g:

diff --git a/kernel/sched.c b/kernel/sched.c
index 2d1e23a..2e0ba74 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3819,6 +3819,8 @@ static void __wake_up_common(wait_queue_head_t *q, 
unsigned int mode,
  {
  	wait_queue_t *curr, *next;

+	ipipe_check_context(ipipe_root_domain);
+
  	list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
  		unsigned flags = curr->flags;

At any rate, you should not expect the system to help you getting away 
with violations of basic dual kernel programming rules from kernel 
space, this won't happen.

>
> Anyhow, I am as impressed as always with this piece of software. This is the toolkit that any embedded developer needs. And RTDM has been a great addition.
>
> thanks
> Jorge
>
> ________________________________________
> From: Gilles Chanteperdrix [gilles.chanteperdrix@xenomai.org]
> Sent: 12 July 2012 09:31
> To: Jorge Ramirez Ortiz,  HCL Europe
> Cc: xenomai@xenomai.org
> Subject: Re: [Xenomai] BUG: Unhandled exception over domain Xenomai - switching to ROOT
>
> On 07/12/2012 10:16 AM, Jorge Ramirez Ortiz, HCL Europe wrote:
>> Thanks Gilles. Yes unfortunately I cant recompile this kernel so I
>> was wondering about the system context of this fault within Xenomai.
>> BTW how come the stack frame is not printed by default.
>
> Because it may not be safe to use show_stack() from RT context (for
> instance, the print_symbol function to print symbol names need to take a
> spinlock when a symbol is defined by a kernel module, and this spinlock
> is not rt-safe). Anyway, the error message normally prints the PC of the
> error.
>
> Could you give us the exact error messsage? The version of Xenomai, the
> Linux kernel, the version of the I-pipe patch you use?
>
> --
>                                              Gilles.
>
>
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-- 
Philippe.