linux-kernel - Re: [RFC PATCH 8/9] livepatch: allow patch modules to be removed

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Message-ID: <20150210195706.GB1118@treble.redhat.com>
Date:	Tue, 10 Feb 2015 13:57:06 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Seth Jennings <sjenning@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 8/9] livepatch: allow patch modules to be removed

On Tue, Feb 10, 2015 at 08:02:34PM +0100, Jiri Slaby wrote:
> On 02/09/2015, 06:31 PM, Josh Poimboeuf wrote:
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> ...
> > @@ -497,10 +500,6 @@ static struct attribute *klp_patch_attrs[] = {
> >  
> >  static void klp_kobj_release_patch(struct kobject *kobj)
> >  {
> > -	/*
> > -	 * Once we have a consistency model we'll need to module_put() the
> > -	 * patch module here.  See klp_register_patch() for more details.
> > -	 */
> 
> I deliberately let you write the note in there :). What happens when I
> leave some attribute in /sys open and you remove the module in the meantime?

You're right, as was I the first time :-)

The only problem is that it would be nice if we could call
klp_unregister_patch() from the patch module's exit function, so that
doing an rmmod on the patch module unregisters it.  But if we put
module_put() in the patch release function, then we have a circular
dependency and we could never rmmod it.

How about instead we do a klp_is_patch_registered() at the beginning of
all the attribute accessor functions?  It's kind of ugly, but I can't
think of a better idea at the moment.

-- 
Josh
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