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Date: Sat, 2 Jul 2011 11:39:29 +0530
From: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
To: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Frederic Weisbecker <fweisbec@gmail.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
        yrl.pp-manager.tt@hitachi.com
Subject: Re: [RFC PATCH -tip ] [BUGFIX] x86: Remove preempt disabling from
	kprobes
Message-ID: <20110702060929.GA5511@in.ibm.com>
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On Fri, Jul 01, 2011 at 10:14:08PM +0900, Masami Hiramatsu wrote:
> Steven Rostedt reported that putting kprobe on the instruction which
> loads preempt_count causes the wrong result, as below.
> 
> Kprobes requires preemption to be disabled as it single steps the code
> it replaced with a breakpoint. But because the code that is single
> stepped could be reading the preempt count, the kprobe disabling of the
> preempt count can cause the wrong value to end up as a result. Here's an
> example:

...

> We just caused preempt count to increment twice when it should have only
> incremented once, and this screws everything else up.
> 
> To solve this, I've removed preempt disabling code from kprobes,
> since the breakpoint exception and kprobes single step routine
> disables interrupts, it doesn't need to disable preemption while
> single-stepping anymore.
> 
> This patch is for -tip tree, and it can be applied to linus tree too.
> 
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Reported-by: Steven Rostedt <rostedt@goodmis.org>
> ---

Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>