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

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2014/05/17 1:27), 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);
> 		}
> 	}

In this case, I'd recommend you to add foo() to replacing target
as dummy. Then, kpatch can ensure foo() is actually not running. :)

> 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).
> 
> With lazy-switching implemented in kgraft, this can never happen.

And I guess similar thing may happen with kgraft. If old function and
new function share a non-auto variable and they modify it different way,
the result will be unexpected by the mutual interference.

Thank you,

> 
> 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,
> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com