linux-kernel - Re: [PATCH 1/5] ftrace: Synchronize variable setting with breakpoints

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Message-ID: <1338487413.28384.103.camel@twins>
Date:	Thu, 31 May 2012 20:03:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Jones <davej@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/5] ftrace: Synchronize variable setting with
 breakpoints

On Thu, 2012-05-31 at 13:53 -0400, Steven Rostedt wrote:
> On Thu, 2012-05-31 at 19:40 +0200, Peter Zijlstra wrote:
> > On Thu, 2012-05-31 at 10:08 -0400, Steven Rostedt wrote:
> > > > Also, why does this stuff live in ftrace? I always thought you were
> > > > going to replace text_poke() so everybody that uses cross-modifying code
> > > > could profit?
> > > 
> > > I discussed this with Masami at Collaboration Summit. The two are
> > > similar but also very different. But we want to start merging the two
> > > together where it makes sense. 
> > 
> > Argh,. I so disagree. You're doing it backwards.
> > 
> > First you merge whatever is there, regardless of who came first.
> 
> The comment about coming first was more about 're-inventing' then about
> merging. You can't reinvent something that didn't exist.
> 
> That said, I didn't even think about text poke while doing this.

Well, the fail is before that, how could we grow two pieces of code
doing similar things in the first place? 

>  I was
> just simply thinking about removing stop_machine from ftrace, that
> required this. It was only a after thought that text_poke() could do the
> same. And this came up at Collab, where I thought, oh yeah! we can
> incorporate this with text poke.

But but but but.. the thing far back when Mathieu proposed the int3
scheme it was text_poke().. how.. did you not think of it this time!?

> 
> > Then, when everybody doing text modification is using the same
> > interface, do a second implementation using a Kconfig knob. If the scary
> > new one breaks, no sweat, flip the config. If its proven stable, kill
> > off the old one.
> 
> What do you suggest then? To revert the code and rewrite it so that
> text_poke() does a similar thing?

Too late for that now I guess.. I just wonder why you all thought it was
a good idea to have two pieces of code doing cross-modifying-code. I
always assumed ftrace used text_poke().

I hardly ever use dyn-ftrace but I do use some text_poke() through
jump_labels.

I would still like to end up with one code base doing CMC with two
implementations depending on a Kconfig knob.
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