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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, May 16, 2014 at 06:27:27PM +0200, Jiri Kosina wrote:
> On Tue, 6 May 2014, Steven Rostedt wrote:
> 
> > > However, I also think if users can accept such freezing wait-time,
> > > it means they can also accept kexec based "checkpoint-restart" patching.
> > > So, I think the final goal of the kpatch will be live patching without
> > > stopping the machine. I'm discussing the issue on github #138, but that is
> > > off-topic. :)
> > 
> > I agree with Ingo too. Being conservative at first is the right
> > approach here. We should start out with a stop_machine making sure that
> > everything is sane before we continue. Sure, that's not much different
> > than a kexec, but lets take things one step at a time.
> > 
> > ftrace did the stop_machine (and still does for some archs), and slowly
> > moved to a more efficient method. kpatch/kgraft should follow suit.
> 
> I don't really agree here.
> 
> I actually believe that "lazy" switching kgraft is doing provides a little 
> bit more in the sense of consistency than stop_machine()-based aproach.
> 
> 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()?

> 
> With lazy-switching implemented in kgraft, this can never happen.
> 
> So I'd like to ask for a little bit more explanation why you think the 
> stop_machine()-based patching provides more sanity/consistency assurance 
> than the lazy switching we're doing.
> 
> Thanks a lot,
> 
> -- 
> Jiri Kosina
> SUSE Labs

-- 
Josh