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

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Date:	Thu, 19 Feb 2015 10:52:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vojtech Pavlik <vojtech@...e.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Jiri Kosina <jkosina@...e.cz>,
	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>
Subject: Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

On Wed, Feb 18, 2015 at 09:44:44PM +0100, Vojtech Pavlik wrote:
> For live patching it doesn't matter whether code is running, sleeping or
> frozen.
> 
> What matters is whether there is state before patching that may not be
> valid after patching.
> 
> For userspace tasks, the exit from a syscall is a perfect moment for
> switching to the "after" state, as all stacks, and thus all local
> variables are gone and no local state exists in the kernel for the
> thread.
> 
> The freezer is a logical choice for kernel threads, however, given that
> kernel threads have no defined entry/exit point and execute within a
> single main function, local variables stay and thus local state persists
> from before to after freezing.
> 
> Defining that no local state within a kernel thread may be relied upon
> after exiting from the freezer is certainly possible, and is already
> true for many kernel threads.
> 
> It isn't a given property of the freezer itself, though. And isn't
> obvious for author of new kernel threads either.
> 
> The ideal solution would be to convert the majority of kernel threads to
> workqueues, because then there is a defined entry/exit point over which
> state isn't transferred. That is a lot of work, though, and has other
> drawbacks, particularly in the realtime space.

kthread_park() functionality seems to be exactly what you want.
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