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

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Date:	Mon, 5 May 2014 08:26:38 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Seth Jennings <sjenning@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.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 Mon, May 05, 2014 at 10:55:37AM +0200, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > [...]
> > 
> > kpatch checks the backtraces of all tasks in stop_machine() to 
> > ensure that no instances of the old function are running when the 
> > new function is applied.  I think the biggest downside of this 
> > approach is that stop_machine() has to idle all other CPUs during 
> > the patching process, so it inserts a small amount of latency (a few 
> > ms on an idle system).
> 
> When live patching the kernel, how about achieving an even 'cleaner' 
> state for all tasks in the system: to freeze all tasks, as the suspend 
> and hibernation code (and kexec) does, via freeze_processes()?
> 
> That means no tasks in the system have any real kernel execution 
> state, and there's also no problem with long-sleeping tasks, as 
> freeze_processes() is supposed to be fast as well.
> 
> I.e. go for the most conservative live patching state first, and relax 
> it only once the initial model is upstream and is working robustly.

I had considered doing this before, but the problem I found is that many
kernel threads are unfreezable.  So we wouldn't be able to check whether
its safe to replace any functions in use by those kernel threads.

-- 
Josh
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