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

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Message-ID: <20150212154852.GF18578@treble.redhat.com>
Date:	Thu, 12 Feb 2015 09:48:52 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>, Vojtech Pavlik <vojtech@...e.cz>
Subject: Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

On Thu, Feb 12, 2015 at 02:26:42PM +0100, Jiri Slaby wrote:
> On 02/12/2015, 04:21 AM, Josh Poimboeuf wrote:
> > Ingo, Peter,
> > 
> > Would you have any objections to making task_rq_lock/unlock() non-static
> > (or moving them to kernel/sched/sched.h) so they can be called by the
> > livepatch code?
> > 
> > To provide some background, I'm looking for a way to temporarily prevent
> > a sleeping task from running while its stack is examined, to decide
> > whether it can be safely switched to the new patching "universe".  For
> > more details see klp_transition_task() in the patch below.
> > 
> > Using task_rq_lock() is the most straightforward way I could find to
> > achieve that.
> 
> Hi, I cannot speak whether it is the proper way or not.
> 
> But if so, would it make sense to do the opposite: expose an API to walk
> through the processes' stack and make the decision? Concretely, move
> parts of klp_stacktrace_address_verify_func to sched.c or somewhere in
> kernel/sched/ and leave task_rq_lock untouched.

Yeah, it makes sense in theory.  But I'm not sure how to do that in a
way that prevents races when switching the task's universe.  I think we
need the rq locked for both the stack walk and the universe switch.

In general, I agree it would be good to find a way to keep the rq
locking functions in sched.c.

-- 
Josh
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