linux-kernel - Re: [PATCH -tip 0/4] Text edit lock and atomic text

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Date:	Fri, 6 Mar 2009 13:09:11 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <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>
Subject: Re: [PATCH -tip 0/4] Text edit lock and atomic text_poke()

* Masami Hiramatsu (mhiramat@...hat.com) wrote:
> Hi,
> 
> Here is a series of patches which introduce text_mutex for protecting
> editing kernel text from each other subsystems, and make text_poke()
> atomic by using fixmap.
> 
> BTW,
> 
> > Paravirt and alternatives are always done when SMP is inactive, so there is no
> > need to use locks.
> 
> Mathieu, I'm not sure that means. alternatives will be called from module
> init code and other place where the system has already been running multi-
> -threads(and they use smp_alt mutex). So, is it possible that those functions
> will sleep or yield to another process?
> 

As I remember, module code uses text_poke_early when modifying the
module text _before_ the module is made available to the rest of the
kernel. Therefore, it behaves as if it was in a UP system, and that's
ok, because only one CPU has to touch the text. Therefore, there is no
need to use text_poke, and therefore no need to take the text mutex.

Or maybe has it changed ? But I doubt so.

Mathieu

> Anyway, I added a new patch which locks text_mutex in alternative_smp_*.
> 
> Thank you,
> 
>  arch/x86/include/asm/fixmap.h |    2 ++
>  arch/x86/kernel/alternative.c |   29 ++++++++++++++++++++---------
>  include/linux/memory.h        |    6 ++++++
>  kernel/kprobes.c              |   15 +++++++++++++--
>  mm/memory.c                   |    9 +++++++++
>  5 files changed, 50 insertions(+), 11 deletions(-)
> 
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@...hat.com
> 

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