Philippe Gerum wrote:
> On Wed, 2010-06-02 at 11:37 +0200, Gilles Chanteperdrix wrote:
>> Philippe Gerum wrote:
>>> On Wed, 2010-06-02 at 11:20 +0200, Jan Kiszka wrote:
>>>> Philippe Gerum wrote:
>>>>> On Wed, 2010-06-02 at 10:36 +0200, Gilles Chanteperdrix wrote:
>>>>>> Jan Kiszka wrote:
>>>>>>> Tschaeche IT-Services wrote:
>>>>>>>> On Tue, Jun 01, 2010 at 04:32:37PM +0200, Philippe Gerum wrote:
>>>>>>>>> Not in the absence of syscall. We thought about this once already,
>>>>>>>>> when
>>>>>>>>> considering how a watchdog preempting a runaway task in primary mode
>>>>>>>>> could force a secondary mode switch: there is no sane and easy
>>>>>>>>> solution
>>>>>>>>> to this unfortunately.
>>>>>>>> This is exactly Sigmatek's problem: Our customers develop code
>>>>>>>> within our debugging/development environment. We want to catch
>>>>>>>> this situation (the developer implements a while(1)) with a
>>>>>>>> watchdog throwing SIGTRAP so that our debugger gets active
>>>>>>>> and can locate the problem according to the stack frame...
>>>>>>> CONFIG_XENO_OPT_WATCHDOG is probably what you are looking for. It tries
>>>>>>> to catch "well-behaving" broken threads via SIGDEBUG and kills the
>>>>>>> hopelessly broken rest - system alive again.
>>>>>>>
>>>>>>> You can then debug the former and need to do code review on the latter.
>>>>>>> Or you could also try to add some loop-breaking Xenomai syscalls (or
>>>>>>> even more clever checks) to library services the code under suspect
>>>>>>> usually invokes.
>>>>>> I am afraid "well-behaving" means emitting syscalls. We have a radical
>>>>>> way to cause a SIGSEGV to be sent to a thread having run amok: set its
>>>>>> PC to an invalid address (after having printed the real PC). gdb will
>>>>>> not be able to print where the program stopped, but should be able to
>>>>>> print the backtrace.
>>>>>>
>>>>> Actually, we could extend this logic and forge a stack frame to return
>>>>> to the preempted application code via some userland trampoline code,
>>>>> doing the switch:
>>>>>
>>>>> [watchdog trigger]
>>>>> forge_return_frame(on =regs->sp, to =regs->pc);
>>>>> regs->pc = __oops_I_did_it_again;
>>>>>
>>>>> __oops_I_did_it_again:
>>>>> __xn_migrate(LINUX_DOMAIN);
>>>>> ret (via forged frame)
>>>> Yep, that's what came to my mind as well. But the __oops_I_did_it_again
>>>> part has to reside in user space, no?
>>> Clearly, yes. Either we map this explictly, or we just make sure to
>>> compile it in each app, and pass its address at skin binding time. Our
>>> text is mmlocked anyway.
>>>
>>>>> The thing is, that this brings in some arch-dep code to forge a stack
>>>>> frame (like the kernel uses for signals), that should rather live in the
>>>>> pipeline core.
>>>> Actually, we are then close to enabling signal delivery outside syscalls...
>>>>
>>> Yes, looks like.
>> When thinking about this real signals things, I was thinking about
>> putting the forging code into Xenomai (the code is the same for all
>> kernel versions, so there is no reason to put it into the I-pipe, and we
>> may have to emit a special syscall to restore the context when handling
>> the signal is done). What we need the I-pipe for, however, is to trigger
>> some event on the way back to user-space.
>>
>
> A reason to have this code in the pipeline core is because we would
> duplicate the setup_rt_frame code already available from the vanilla
> kernel. It's a bit like xnarch_switch_to: we used to open code most of
> it in our arch-dep code, mostly duplicating the vanilla switch code, but
> having switch_mm() ironed enough - on arm and powerpc at least - to be
> callable from the Xenomai domain as well proved to be a serious relief.
>
> Granted, the signal code is unlikely to change a lot, given the strong
> ABI requirements this has wrt the glibc, but I'm always reluctant to
> introduce duplicates at both ends of the system; I would rather factor
> out that code and make it available to both domains, if that makes
> sense.
I am not sure it really makes sense: the biggest part of the linux code
is used to setup the special frame passed as the last void * pointer of
signal handlers with the SA_SIGINFO option, allowing (among others)
signal handlers to use setcontext() to implement co-routines, and I am
not sure we really want that. And if you do some major revamping of
Linux stack frame build functions, you will have merge conflicts every
time you upgrade the I-pipe patch.
Besides, we still have the return through syscall issue: returning from
the signal handler can not be a simple "return" instruction, since we
have to save and restore most registers.
--
Gilles.
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