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

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Date:	Fri, 6 Mar 2009 13:12:19 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	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()


On Fri, 6 Mar 2009, Mathieu Desnoyers wrote:

> * 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.
> 

That is correct. Currently ftrace simply does a probe_kernel_write on 
module code to convert the calls to mcount into nops. This is done before 
the module is available, and does not require stop_machine nor break 
points. Since modules are not included in the DEBUG_RODATA there's no need 
to play games with the page tables either.

-- Steve

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