Re: [PATCH tracing/kprobes] kprobes: Disable booster when CONFIG_PREEMPT=y

[Mathieu Desnoyers <compudj at krystal dot dyndns dot org>]

* Peter Zijlstra (peterz@infradead.org) wrote:
> On Fri, 2010-01-29 at 12:08 -0500, Mathieu Desnoyers wrote:
> > 
> > If a task is set as stopped, and the preempted before calling schedule,
> > can this result in a preempted task staying in that state for an
> > arbitrary long period of time ? Or is there some mechanism prohibiting
> > that in the scheduler ? 
> 
> PREEMPT_ACTIVE does that:
> 
> preempt_schedule()
>                 add_preempt_count(PREEMPT_ACTIVE);
>                 schedule();
> 
> 
> schedule()
>         if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
>                 if (unlikely(signal_pending_state(prev->state, prev)))
>                         prev->state = TASK_RUNNING;
>                 else
>                         deactivate_task(rq, prev, 1);
>                 switch_count = &prev->nvcsw;
>         }

OK, it looks safe for preemption. Is there any unforeseen weird way a
task can be scheduled out and stopped that would permit it to either:

- stall the algorithm forever (DoS)
- appear as quiescent to the algorithm while its stack would hold return
  pointers to incorrect locations

?

I'm concerned about page faults here.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
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