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

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Date:	Sat, 17 May 2014 09:10:17 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Josh Poimboeuf <jpoimboe@...hat.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, Steven Rostedt wrote:

> Why I still favor the stop_machine approach, is because the method of
> patching is a bit simpler that way. A "lazy" approach will be more
> complex and more likely to be buggy. The thing I'm arguing here is not
> the end result being a problem, but the implementation of the patching
> itself causing bugs.

Well, what can I say to this.

	21 files changed, 594 insertions(+), 10 deletions(-)

that's a complete implementation, including comments and some 
documentation.

Yes, it still has TODOs (such as patching modules as they are modprobed, 
we're working on multi-arch support, etc), but it's more or less complete 
working x86 skeleton.

> I rather have a "lazy" approach, 

I'm glad to hear that, thanks :)

> but like ftrace and its breakpoint method, the stop_machine approach is 
> the simpler way to make sure the patching works before we try to 
> optimize it.

I am still not convinced that it's more complex. It's actually lazy both 
in the way it performs patching and in implementation -- we basically set 
a flag, flip the switch, and let the universe converge to a new state by 
itself.

It's basically hard to argue about level of bugginess when no actual bugs 
are being pointed out :) (well, yes, the kthreads stuff needs to be taken 
care of, but both kgraft and kpatch have similar issues there).

Thanks,

-- 
Jiri Kosina
SUSE Labs
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