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

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Date:	Mon, 2 Mar 2009 19:00:27 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	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 (mingo@...e.hu) 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.
> 
> 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

This is why I prepared 

text-edit-lock-architecture-independent-code.patch
text-edit-lock-kprobes-architecture-independent-support.patch

A while ago. I'll post them right away.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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