From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4C06265C.3030108@domain.hid> References: <20100601135005.GA5483@domain.hid> <1275402757.27918.151.camel@domain.hid> <20100601155403.GA8240@domain.hid> <4C053C51.4090903@domain.hid> <4C061823.70005@domain.hid> <1275470136.18250.16.camel@domain.hid> <4C062246.40107@domain.hid> <1275470925.18250.18.camel@domain.hid> <4C06265C.3030108@domain.hid> Content-Type: text/plain; charset="UTF-8" Date: Wed, 02 Jun 2010 12:06:14 +0200 Message-ID: <1275473174.18250.36.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-help] Handling Linux Signals in primary domain context List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: Jan Kiszka , "xenomai@xenomai.org" 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. -- Philippe.