linux-kernel - Re: [RFC][PATCH] x86: make text

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Date:	Mon, 02 Mar 2009 19:05:39 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC][PATCH] x86: make text_poke() atomic



Ingo Molnar wrote:
> * Masami Hiramatsu <mhiramat@...hat.com> wrote:
> 
>> Ingo Molnar wrote:
>>>>> So perhaps another approach to (re-)consider would be to go back 
>>>>> to atomic fixmaps here. It spends 3 slots but that's no big 
>>>>> deal.
>>>> Oh, it's a good idea! fixmaps must make it simpler.
>>>>
>>>>> In exchange it will be conceptually simpler, and will also scale 
>>>>> much better than a global spinlock. What do you think?
>>>> I think even if I use fixmaps, we have to use a spinlock to protect
>>>> the fixmap area from other threads...
>>> that's why i suggested to use an atomic-kmap, not a fixmap.
>> Even if the mapping is atomic, text_poke() has to protect pte
>> from other text_poke()s while changing code.
>> AFAIK, atomic-kmap itself doesn't ensure that, does it?
> 
> Well, but text_poke() is not a serializing API to begin with. 
> It's normally used in code patching sequences when we 'know' 
> that there cannot be similar parallel activities. The kprobes 
> usage of text_poke() looks unsafe - and that needs to be fixed.

Oh, kprobes already prohibited parallel arming/disarming
by using kprobe_mutex. :-)

> So indeed a new global lock is needed there.
> 
> It's fixable and we'll fixit, but text_poke() is really more 
> complex than i'd like it to be.
> 
> stop_machine_run() is essentially instantaneous in practice and 
> obviously serializing so it warrants a second look at least. 
> Have you tried to use it in kprobes?

No, but it seems that cost high for incremental use(registration)
of kprobes...

Thank you,

> 
> 	Ingo

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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