From: Philippe Gerum <rpm@xenomai.org>
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Date: Thu, 03 Jun 2010 10:27:26 +0200
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Subject: Re: [Xenomai-core] [RFC] Break out of endless user space loops
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To: Jan Kiszka <jan.kiszka@domain.hid>
Cc: xenomai-core <xenomai@xenomai.org>, Tschaeche IT-Services <services@domain.hid>

On Thu, 2010-06-03 at 08:55 +0200, Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
> > Jan Kiszka wrote:
> >> Hi all,
> >>
> >> here is the first apparently working prototype for getting hold of
> >> endless user space loops in RT threads. A simple test case of mine now
> >> receive a SIGDEBUG even if it does "while (1);".
> >>
> >> The design follows Gilles' suggestion to force a SEGV on victim thread
> >> but restore the patched PC before migrating the thread after this fault.
> >> The only drawback of this approach: We need to keep track of the
> >> preempted register set at I-pipe level. I basically replicated what
> >> Linux does these days as well and exported it as ipipe_get_irq_regs()
> >> (the second patch).
> > 
> > You already have the regs in xnarch_fault_info.
> > 
> 
> We only pass this around for exceptions.

And for a good reason, exceptions are always delivered synchronously
upon receipt, not IRQs, given the deferred dispatching scheme. Your
ipipe_get_irq_regs interface is inherently broken for anything which is
not a wired-mode timer IRQ, since you could pass the caller a reference
to an unwound stack frame.

You have to resort to __ipipe_tick_regs, and obviously only use this in
the context of a timer-triggered code, like the watchdog handler, which
saves your day.

> 
> Jan
> 
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-- 
Philippe.