linux-kernel - Re: module notifier: was Re: [PATCH 2/2] kernel: add support for live patching

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Date:	Tue, 11 Nov 2014 13:40:08 -0600
From:	Seth Jennings <sjenning@...hat.com>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	live-patching@...r.kernel.org, kpatch@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: module notifier: was Re: [PATCH 2/2] kernel: add support for
 live patching

On Fri, Nov 07, 2014 at 07:40:11PM +0100, Petr Mladek wrote:
> On Fri 2014-11-07 12:07:11, Seth Jennings wrote:
> > On Fri, Nov 07, 2014 at 06:13:07PM +0100, Petr Mladek wrote:
> > > On Thu 2014-11-06 08:39:08, Seth Jennings wrote:
[...]
> > > > +	up(&lpc_mutex);
> > > > +	WARN("failed to apply patch '%s' to module '%s'\n",
> > > > +		patch->mod->name, mod->name);
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static struct notifier_block lp_module_nb = {
> > > > +	.notifier_call = lp_module_notify,
> > > > +	.priority = INT_MIN, /* called last */
> > > 
> > > The handler for MODULE_STATE_COMMING would need have higger priority,
> > > if we want to cleanly unregister the ftrace handlers.
> > 
> > Yes, we might need two handlers at different priorities if we decide to
> > go that direction: one for MODULE_STATE_GOING at high/max and one for
> > MODULE_STATE_COMING at low/min.
> 
> kGraft has notifier only for the going state. The initialization is
> called directly from load_module() after ftrace_module_init()
> and complete_formation() before it is executed by parse_args().
> 
> I need to investigate if the notifier is more elegant and safe or not.

I looked it up and having a COMING notifier with priority INT_MIN is
effectively the same as having a call between complete_formation() and
parse_args() since the notifiers are called as the last thing in
complete_formation().

I think I've found a clean way to avoid the ref taking on the patched
modules using only the notifier and lpc_mutex. It will be in v2
(hopefully out in the next couple of days).

Thanks,
Seth

> 
> Best Regards,
> Petr
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