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

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Date:	Wed, 11 Feb 2015 12:39:49 -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 Wed, Feb 11, 2015 at 11:55:05AM +0100, Jiri Slaby wrote:
> On 02/10/2015, 08:57 PM, Josh Poimboeuf wrote:
> > 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.
> 
> Ugh, no :). You even have the kobject proper in the module which would
> be gone.
> 
> However we can take inspiration in kgraft. I introduced a completion
> there and wait for it in rmmod. This completion is made complete in
> kobject's release. See:
> https://git.kernel.org/cgit/linux/kernel/git/jirislaby/kgraft.git/tree/kernel/kgraft_files.c?h=kgraft#n30
> https://git.kernel.org/cgit/linux/kernel/git/jirislaby/kgraft.git/tree/kernel/kgraft_files.c?h=kgraft#n138
> 
> This should IMO work here too.

Thanks, that sounds a lot better.  I'll try to do something like that.

-- 
Josh
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