On Tue, 2011-07-12 at 14:57 +0200, Jan Kiszka wrote:
> On 2011-07-12 14:13, Jan Kiszka wrote:
> > On 2011-07-12 14:06, Gilles Chanteperdrix wrote:
> >> On 07/12/2011 01:58 PM, Jan Kiszka wrote:
> >>> On 2011-07-12 13:56, Jan Kiszka wrote:
> >>>> However, this parallel unsynchronized execution of the gatekeeper and
> >>>> its target thread leaves an increasingly bad feeling on my side. Did we
> >>>> really catch all corner cases now? I wouldn't guarantee that yet.
> >>>> Specifically as I still have an obscure crash of a Xenomai thread on
> >>>> Linux schedule() on my table.
> >>>>
> >>>> What if the target thread woke up due to a signal, continued much
> >>>> further on a different CPU, blocked in TASK_INTERRUPTIBLE, and then the
> >>>> gatekeeper continued? I wish we could already eliminate this complexity
> >>>> and do the migration directly inside schedule()...
> >>>
> >>> BTW, we do we mask out TASK_ATOMICSWITCH when checking the task state in
> >>> the gatekeeper? What would happen if we included it (state ==
> >>> (TASK_ATOMICSWITCH | TASK_INTERRUPTIBLE))?
> >>
> >> I would tend to think that what we should check is
> >> xnthread_test_info(XNATOMIC). Or maybe check both, the interruptible
> >> state and the XNATOMIC info bit.
> >
> > Actually, neither the info bits nor the task state is sufficiently
> > synchronized against the gatekeeper yet. We need to hold a shared lock
> > when testing and resetting the state. I'm not sure yet if that is
> > fixable given the gatekeeper architecture.
> >
>
> This may work (on top of the exit-race fix):
>
> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c
> index 50dcf43..90feb16 100644
> --- a/ksrc/nucleus/shadow.c
> +++ b/ksrc/nucleus/shadow.c
> @@ -913,20 +913,27 @@ static int gatekeeper_thread(void *data)
> if ((xnthread_user_task(target)->state & ~TASK_ATOMICSWITCH) ==
> TASK_INTERRUPTIBLE) {
> rpi_pop(target);
> xnlock_get_irqsave(&nklock, s);
> -#ifdef CONFIG_SMP
> +
> /*
> - * If the task changed its CPU while in
> - * secondary mode, change the CPU of the
> - * underlying Xenomai shadow too. We do not
> - * migrate the thread timers here, it would
> - * not work. For a "full" migration comprising
> - * timers, using xnpod_migrate_thread is
> - * required.
> + * Recheck XNATOMIC to avoid waking the shadow if the
> + * Linux task received a signal meanwhile.
> */
> - if (target->sched != sched)
> - xnsched_migrate_passive(target, sched);
> + if (xnthread_test_info(target, XNATOMIC)) {
> +#ifdef CONFIG_SMP
> + /*
> + * If the task changed its CPU while in
> + * secondary mode, change the CPU of the
> + * underlying Xenomai shadow too. We do not
> + * migrate the thread timers here, it would
> + * not work. For a "full" migration comprising
> + * timers, using xnpod_migrate_thread is
> + * required.
> + */
> + if (target->sched != sched)
> + xnsched_migrate_passive(target, sched);
> #endif /* CONFIG_SMP */
> - xnpod_resume_thread(target, XNRELAX);
> + xnpod_resume_thread(target, XNRELAX);
> + }
> xnlock_put_irqrestore(&nklock, s);
> xnpod_schedule();
> }
> @@ -1036,6 +1043,7 @@ redo:
> * to process this signal anyway.
> */
> if (rthal_current_domain == rthal_root_domain) {
> + XENO_BUGON(NUCLEUS, xnthread_test_info(thread, XNATOMIC));
> if (XENO_DEBUG(NUCLEUS) && (!signal_pending(this_task)
> || this_task->state != TASK_RUNNING))
> xnpod_fatal
> @@ -1044,6 +1052,8 @@ redo:
> return -ERESTARTSYS;
> }
>
> + xnthread_clear_info(thread, XNATOMIC);
> +
> /* "current" is now running into the Xenomai domain. */
> thread->gksched = NULL;
> sched = xnsched_finish_unlocked_switch(thread->sched);
> @@ -2650,6 +2660,8 @@ static inline void do_sigwake_event(struct task_struct
> *p)
>
> xnlock_get_irqsave(&nklock, s);
>
> + xnthread_clear_info(thread, XNATOMIC);
> +
> if ((p->ptrace & PT_PTRACED) && !xnthread_test_state(thread, XNDEBUG)) {
> sigset_t pending;
>
>
> It totally ignores RPI and PREEMPT_RT for now. RPI is broken anyway,
I want to drop RPI in v3 for sure because it is misleading people. I'm
still pondering whether we should do that earlier during the 2.6
timeframe.
> ripping it out would allow to use solely XNATOMIC as condition in the
> gatekeeper.
>
> /me is now looking to get this applied to a testbox that still crashes
> suspiciously.
>
> Jan
>
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--
Philippe.
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