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

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Message-ID: <20160407180302.c5x3sj6b3ditizc4@treble.redhat.com>
Date:	Thu, 7 Apr 2016 13:03:02 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Kosina <jikos@...nel.org>
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, Apr 07, 2016 at 05:47:00PM +0200, Jiri Kosina wrote:
> On Thu, 7 Apr 2016, Josh Poimboeuf wrote:
> 
> > > > - try ftrace handler switching idea from v1 cover letter
> [ ... ]
> > > We probably should not check the stack in atomic context
> > 
> > Can you elaborate why not?
> 
> 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]:

| 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?

[1] https://lkml.kernel.org/r/cover.1423499826.git.jpoimboe@redhat.com:

-- 
Josh