linux-kernel - Re: [BUG] kprobes crashing because of preempt count

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Message-ID: <1309484007.26417.121.camel@gandalf.stny.rr.com>
Date:	Thu, 30 Jun 2011 21:33:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, yrl.pp-manager.tt@...achi.com
Subject: Re: [BUG] kprobes crashing because of preempt count

On Fri, 2011-07-01 at 10:12 +0900, Masami Hiramatsu wrote:

> > Do we really need to have preemption disabled throughout this? Is it
> > because we don't want to migrate or call schedule? Not sure what the
> > best way to fix this is. Perhaps we add a kprobe_preempt_disable() that
> > is checked as well?
> 
> I think the best way to do that is just removing preemption disabling
> code, because
> - breakpoint exception itself disables interrupt (at least on x86)
> - While single stepping, interrupts also be disabled.

I guess the above point is critical. If interrupts are disabled through
out the entire walk through, then we are fine, as that just guarantees
preemption is disabled anyway. But! if it does get enabled anywhere,
then we will have issues as the two traps require using the same state
data that is stored per cpu.

> (BTW, theoretically, boosted and optimized kprobes shouldn't have
> this problem, because those doesn't execute single-stepping)

Does the optimized kprobes even disable preemption?

> 
> So, I think there is no reason of disabling preemption.

That would be the best solution.

-- Steve


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