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

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Message-ID: <20090302234910.GA17956@elte.hu>
Date:	Tue, 3 Mar 2009 00:49:10 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Masami Hiramatsu <mhiramat@...hat.com>
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


* 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.

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?

	Ingo
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