linux-kernel - Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

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Date:	Thu, 12 Feb 2015 13:39:51 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>
Subject: Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

On Thu, 12 Feb 2015, Peter Zijlstra wrote:

> > > FWIW, the barriers in klp_update_task_universe() and 
> > > klp_set_universe_goal() look like complete crack, and their comments are 
> > > seriously deficient.
> > 
> > These particular barriers seem correct to me; you basically need to make 
> > sure that whenever a thread with TIF_KLP_NEED_UPDATE goes through 
> > do_notify_resume(), it sees proper universe number to be converted to.
> 
> I'm not seeing how they're going to help with that.
> 
> The comment should describe the data race and how the barriers are
> making it not happen.
> 
> putting wmb after a store and rmb before a read doesn't avoid the reader
> seeing the old value in any universe I know of.

This is about dependency between klp_universe_goal and TIF_KLP_NEED_UPDATE 
in threadinfo flags.

What is confusing here is that threadinfo flags are not set in 
klp_set_universe_goal() directly, but in the caller 
(klp_start_transition()).

I fully agree with you that this deserves better comment though.

-- 
Jiri Kosina
SUSE Labs
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