Re: [Xenomai-core] [Xenomai-help] Sporadic PC freeze after rt_task_start

[Jan Kiszka <jan.kiszka@domain.hid>]

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Philippe Gerum wrote:
> On Thu, 2007-07-19 at 14:40 +0200, Jan Kiszka wrote:
>> Philippe Gerum wrote:
>>>> And when looking at the holders of rpilock, I think one issue could be
>>>> that we hold that lock while calling into xnpod_renice_root [1], ie.
>>>> doing a potential context switch. Was this checked to be save?
>>> xnpod_renice_root() does no reschedule immediately on purpose, we would
>>> never have been able to run any SMP config more than a couple of seconds
>>> otherwise. (See the NOSWITCH bit).
>> OK, then it's not the cause.
>>
>>>> Furthermore, that code path reveals that we take nklock nested into
>>>> rpilock [2]. I haven't found a spot for the other way around (and I hope
>>>> there is none)
>>> xnshadow_start().
>> Nope, that one is not holding nklock. But I found an offender...
> 
> Gasp. xnshadow_renice() kills us too.

Looks like we are approaching mainline "qualities" here - but they have
at least lockdep (and still face nasty races regularly).

As long as you can't avoid nesting or the inner lock only protects
really, really trivial code (list manipulation etc.), I would say there
is one lock too much... Did I mention that I consider nesting to be
evil? :-> Besides correctness, there is also an increasing worst-case
behaviour issue with each additional nesting level.

Jan


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