From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4C063041.40202@domain.hid> Date: Wed, 02 Jun 2010 12:19:45 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 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> <1275473174.18250.36.camel@domain.hid> In-Reply-To: <1275473174.18250.36.camel@domain.hid> Content-Type: text/plain; charset=UTF-8 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: Philippe Gerum Cc: Jan Kiszka , "xenomai@xenomai.org" 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.