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

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Date:	Fri, 01 Jul 2011 09:19:55 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	ananth@...ibm.com
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	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 18:33 +0530, Ananth N Mavinakayanahalli wrote:
> On Fri, Jul 01, 2011 at 09:01:00PM +0900, Masami Hiramatsu wrote:
> > 
> > Yeah, it seems that same thing is done on arm too. And I'm sure that
> > However, I'm still not sure that entire int3 exec path can run without
> > calling inc_preempt_count.
> > It seems that the function is very primitive, and I doubt we can
> > allow to put kprobes on that...
> 
> Right. I think all preempt manipulation routines need to be __kprobes.
> 

As I stated earlier, it can be triggered on just a read of preempt
count. I would also hate to force preempt_count() reads and updates to
be function calls (that's the only way to make it __kprobes). The
inc/dec preempt count routines are only function calls if debug preempt
or preempt tracing is enabled, which is not the case on most distros.

Not to mention, I think reading the value of preempt count by kprobes is
a legit use case.

> Also, Steve is testing the -rt tree where artefacts related to
> locking/preemption are possibly quite different from the mainline that
> may also be at play here.

Yes, I noticed this first on an -rt variant. But I'm actually testing
patches to move from -rt to mainline. This happens to be more a mainline
issue. Once I noticed this bug, all my tests afterward was based on
v3.0-rc5.

-- Steve

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