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

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Date:	Fri, 1 Jul 2011 17:06:26 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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, Jul 01, 2011 at 10:12:03AM +0900, Masami Hiramatsu wrote:
> (2011/06/30 22:23), Steven Rostedt 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.

On 64-bit powerpc, kprobe handlers are run with interrupts enabled
(MSR_EE = 1), but most instructions (including loads/stores) are
emulated, so for the most part, we don't take the sstep exception.

Ananth
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