linux-kernel - Re: [RFC PATCH v1.9 00/14] livepatch: hybrid consistency model

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Date:	Thu, 7 Apr 2016 20:33:03 +0200 (CEST)
From:	Jiri Kosina <jikos@...nel.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
cc:	Petr Mladek <pmladek@...e.com>, Jessica Yu <jeyu@...hat.com>,
	Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org, Vojtech Pavlik <vojtech@...e.com>
Subject: Re: [RFC PATCH v1.9 00/14] livepatch: hybrid consistency model

On Thu, 7 Apr 2016, Josh Poimboeuf wrote:

> > I admittedly forgot what the "ftrace handler switching idea" is, and am 
> > not sure where exactly to look for it (could you please point it to me so 
> > that I can refresh my memory)
> 
> Here's where I originally described it [1]:

Thanks!

> | 2) As mentioned above, kthreads which are always sleeping on a patched function
> |    will never transition to the new universe.  This is really a minor issue
> |    (less than 1% of patches).  It's not necessarily something that needs to be
> |    resolved with this patch set, but it would be good to have some discussion
> |    about it regardless.
> |    
> |    To overcome this issue, I have 1/2 an idea: we could add some stack checking
> |    code to the ftrace handler itself to transition the kthread to the new
> |    universe after it re-enters the function it was originally sleeping on, if
> |    the stack doesn't already have have any other to-be-patched functions.
> |    Combined with the klp_transition_work_fn()'s periodic stack checking of
> |    sleeping tasks, that would handle most of the cases (except when trying to
> |    patch the high-level thread_fn itself).
> 
> > but generally we can't assume that a memory holding stack of a
> > sleeping task hasn't been reclaimed and wouldn't need to have been
> > paged in again.
> 
> Hm, we're talking about kernel stacks, right?  Are they not always
> resident in memory?

Sure they are, excuse my evening braindamage.

Thanks,

-- 
Jiri Kosina
SUSE Labs