linux-kernel - Re: Re: [PATCH -tip v3 13/23] x86/trap: Use NOKPROBE

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Date:	Wed, 27 Nov 2013 10:38:12 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-arch@...r.kernel.org,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>, x86@...nel.org,
	lkml <linux-kernel@...r.kernel.org>,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
	virtualization@...ts.linux-foundation.org,
	Ingo Molnar <mingo@...hat.com>, systemtap@...rceware.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Seiji Aguchi <seiji.aguchi@....com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: Re: [PATCH -tip v3 13/23] x86/trap: Use NOKPROBE_SYMBOL macro
 in trap.c

(2013/11/23 6:21), Andi Kleen wrote:
> On Wed, Nov 20, 2013 at 04:22:21AM +0000, Masami Hiramatsu wrote:
>> Use NOKPROBE_SYMBOL macro to protect functions from kprobes
>> instead of __kprobes annotation in trap.c.
>> This also applies __always_inline annotation for some cases,
>> because NOKPROBE_SYMBOL() will inhibit inlining by referring
>> the symbol address.
> 
> NOKPROBE_SYMBOL seems to add a reference from some variable to the function?
> 
> With LTO we can optimize away unused functions, but not
> when there are references to the symbol. So this would likely
> prevent optimizations with LTO.

After considering this problem deeper, I decide to say NO against
to your opinion. IMHO, the LTO is just an optimization, on the other
hand, NOKPROBE_SYMBOL is more important because it is for avoiding
kernel crash. I mean, the safety and stability should be prior to
the performance. Anyway, such side-effect of NOKPROBE_SYMBOL looks
very limited. Perhaps, less than 1% of functions will be marked.
How much does it affect for the optimization?

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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