linux-kernel - Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

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Message-ID: <alpine.LNX.2.00.1405201134100.1615@pobox.suse.cz>
Date:	Tue, 20 May 2014 11:37:16 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Seth Jennings <sjenning@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Slaby <jslaby@...e.cz>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

On Fri, 16 May 2014, Josh Poimboeuf wrote:

> > Consider this scenario:
> > 
> > 	void foo()
> > 	{
> > 		for (i=0; i<10000; i++) {
> > 			bar(i);
> > 			something_else(i);
> > 		}
> > 	}
> > 
> > Let's say you want to live-patch bar(). With stop_machine()-based aproach, 
> > you can easily end-up with old bar() and new bar() being called in two 
> > consecutive iterations before the loop is even exited, right? (especially 
> > on preemptible kernel, or if something_else() goes to sleep).
> 
> Can you clarify why this would be a problem?  Is it because the new
> bar() changed some data semantics which confused foo() or
> something_else()?

I guess the example I used wasn't really completely illustrative, sorry 
for that. But I guess this has been answered later in the thread already; 
the thing is that you don't have a complete callgraph available (at least 
I believe you don't ...?), so you don't really know where your patched 
function will be called from, and thus you can't change function arguments 
or return value semantics; with lazy aproach, you can do that.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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