On 2011-07-12 17:48, Philippe Gerum wrote:
> 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.That would only leave us with XNATOMIC being used under PREEMPT-RT for signaling LO_GKWAKE_REQ on schedule out while my patch may clear it on signal arrival. That would prevent a gatekeeper wakeup and lead to a deadlock. I guess we need some separate flag. Anyway, this patch was no magic bullet yet. Our crashes persist. Jan
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