Hi,

well, if I'm not totally wrong, we have a design problem in the
RT-thread hardening path. I dug into the crash Jeroen reported and I'm
quite sure that this is the reason.

So that's the bad news. The good one is that we can at least work around
it by switching off CONFIG_PREEMPT for Linux (this implicitly means that
it's a 2.6-only issue).

@Jeroen: Did you verify that your setup also works fine without
CONFIG_PREEMPT?

But let's start with two assumptions my further analysis is based on:

[Xenomai]
 o Shadow threads have only one stack, i.e. one context. If the
   real-time part is active (this includes it is blocked on some xnsynch
   object or delayed), the original Linux task must NEVER EVER be
   executed, even if it will immediately fall asleep again. That's
   because the stack is in use by the real-time part at that time. And
   this condition is checked in do_schedule_event() [1].

[Linux]
 o A Linux task which has called set_current_state(<blocking_bit>) will
   remain in the run-queue as long as it calls schedule() on its own.
   This means that it can be preempted (if CONFIG_PREEMPT is set)
   between set_current_state() and schedule() and then even be resumed
   again. Only the explicit call of schedule() will trigger
   deactivate_task() which will in turn remove current from the
   run-queue.

Ok, if this is true, let's have a look at xnshadow_harden(): After
grabbing the gatekeeper sem and putting itself in gk->thread, a task
going for RT then marks itself TASK_INTERRUPTIBLE and wakes up the
gatekeeper [2]. This does not include a Linux reschedule due to the
_sync version of wake_up_interruptible. What can happen now?

1) No interruption until we can called schedule() [3]. All fine as we
will not be removed from the run-queue before the gatekeeper starts
kicking our RT part, thus no conflict in using the thread's stack.

3) Interruption by a RT IRQ. This would just delay the path described
above, even if some RT threads get executed. Once they are finished, we
continue in xnshadow_harden() - given that the RT part does not trigger
the following case:

3) Interruption by some Linux IRQ. This may cause other threads to
become runnable as well, but the gatekeeper has the highest prio and
will therefore be the next. The problem is that the rescheduling on
Linux IRQ exit will PREEMPT our task in xnshadow_harden(), it will NOT
remove it from the Linux run-queue. And now we are in real troubles: The
gatekeeper will kick off our RT part which will take over the thread's
stack. As soon as the RT domain falls asleep and Linux takes over again,
it will continue our non-RT part as well! Actually, this seems to be the
reason for the panic in do_schedule_event(). Without
CONFIG_XENO_OPT_DEBUG and this check, we will run both parts AT THE SAME
TIME now, thus violating my first assumption. The system gets fatally
corrupted.

Well, I would be happy if someone can prove me wrong here.

The problem is that I don't see a solution because Linux does not
provide an atomic wake-up + schedule-out under CONFIG_PREEMPT. I'm
currently considering a hack to remove the migrating Linux thread
manually from the run-queue, but this could easily break the Linux
scheduler.

Jan


PS: Out of curiosity I also checked RTAI's migration mechanism in this
regard. It's similar except for the fact that it does the gatekeeper's
work in the Linux scheduler's tail (i.e. after the next context switch).
And RTAI seems it suffers from the very same race. So this is either a
fundamental issue - or I'm fundamentally wrong.


[1]http://www.rts.uni-hannover.de/xenomai/lxr/source/ksrc/nucleus/shadow.c?v=SVN-trunk#L1573
[2]http://www.rts.uni-hannover.de/xenomai/lxr/source/ksrc/nucleus/shadow.c?v=SVN-trunk#L461
[3]http://www.rts.uni-hannover.de/xenomai/lxr/source/ksrc/nucleus/shadow.c?v=SVN-trunk#L481

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