Am 04.11.2010 10:16, Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Take a step back and look at the root cause for this issue again. Unlocked
>>      if need-resched
>>              __xnpod_schedule
>> is inherently racy and will always be (not only for the remote
>> reschedule case BTW).
> Ok, let us examine what may happen with this code if we only set the
> XNRESCHED bit on the local cpu. First, other bits than XNRESCHED do not
> matter, because they can not change under our feet. So, we have two
> cases for this race:
> 1- we see the XNRESCHED bit, but it has been cleared once nklock is
> locked in __xnpod_schedule.
> 2- we do not see the XNRESCHED bit, but it get set right after we test it.
> 1 is not a problem.

Yes, as long as we remove the debug check from the scheduler code (or
fix it somehow). The scheduling code already catches this race.

> 2 is not a problem, because anything which sets the XNRESCHED (it may
> only be an interrupt in fact) bit will cause xnpod_schedule to be called
> right after that.
> So no, no race here provided that we only set the XNRESCHED bit on the
> local cpu.
>  So we either have to accept this and remove the
>> debugging check from the scheduler or push the check back to
>> __xnpod_schedule where it once came from. When this it cleaned up, we
>> can look into the remote resched protocol again.
> The problem of the debug check is that it checks whether the scheduler
> state is modified without the XNRESCHED bit being set. And this is the
> problem, because yes, in that case, we have a race: the scheduler state
> may be modified before the XNRESCHED bit is set by an IPI.
> If we want to fix the debug check, we have to have a special bit, on in
> the sched->status flag, only for the purpose of debugging. Or remove the
> debug check.

Exactly my point. Is there any benefit in keeping the debug check? The
code to make it work may end up as "complex" as the logic it verifies,
at least that's my current feeling.


Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

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