linux-kernel - Re: [RFC][PATCH] kprobes: Add separate preempt

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Message-ID: <1309522789.26417.128.camel@gandalf.stny.rr.com>
Date:	Fri, 01 Jul 2011 08:19:49 -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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH] kprobes: Add separate preempt_disabling for
 kprobes

On Fri, 2011-07-01 at 14:09 +0900, Masami Hiramatsu wrote:

> BTW, on my tip tree, add_preempt_count (a.k.a. inc_preempt_count())
> is marked as __kprobes, so it can not be probed. Is there any change?
> 

That is when debug or preempt tracer is enabled. Otherwise it's
hardcoded into whatever calls it:

#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_PREEMPT_TRACER)
  extern void add_preempt_count(int val);
  extern void sub_preempt_count(int val);
#else
# define add_preempt_count(val)	do { preempt_count() += (val); } while (0)
# define sub_preempt_count(val)	do { preempt_count() -= (val); } while (0)
#endif


Anyway, it still doesn't help the place that reads preempt_count() and
then later uses the result. I hit the crash in schedule_bug() where it
increments preempt count, then reads it to see if it is only 1.

-- Steve


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