linux-kernel - Re: [RFC PATCH 0/9] livepatch: consistency model

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Date:	Fri, 13 Feb 2015 15:22:15 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
cc:	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/9] livepatch: consistency model

On Fri, 13 Feb 2015, Josh Poimboeuf wrote:

> > How about we take a slightly different aproach -- put a probe (or ftrace) 
> > on __switch_to() during a klp transition period, and examine stacktraces 
> > for tasks that are just about to start running from there?
> > 
> > The only tasks that would not be covered by this would be purely CPU-bound 
> > tasks that never schedule. But we are likely in trouble with those anyway, 
> > because odds are that non-rescheduling CPU-bound tasks are also 
> > RT-priority tasks running on isolated CPUs, which we will fail to handle 
> > anyway.
> > 
> > I think Masami used similar trick in his kpatch-without-stopmachine 
> > aproach.
> 
> Yeah, that's definitely an option, though I'm really not too crazy about
> it.  Hooking into the scheduler is kind of scary and disruptive.  

This is basically about running a stack checking for ->next before 
switching to it, i.e. read-only operation (admittedly inducing some 
latency, but that's the same with locking the runqueue). And only when in 
transition phase.

> We'd also have to wake up all the sleeping processes.

Yes, I don't think there is a way around that.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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