linux-kernel - Re: [PATCH tracing/kprobes] kprobes: Disable booster when CONFIG

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Message-ID: <20100129172757.GA4281@Krystal>
Date:	Fri, 29 Jan 2010 12:27:57 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Fr??d??ric Weisbecker" <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>
Subject: Re: [PATCH tracing/kprobes] kprobes: Disable booster when
	CONFIG_PREEMPT=y

* Peter Zijlstra (peterz@...radead.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
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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