linux-kernel - Re: [BUG] Incorrect ordering of klp and ftrace module notifier callbacks

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Date:	Thu, 28 Jan 2016 21:59:13 +0100 (CET)
From:	Jiri Kosina <jikos@...nel.org>
To:	Jessica Yu <jeyu@...hat.com>
cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG] Incorrect ordering of klp and ftrace module notifier
 callbacks

On Thu, 28 Jan 2016, Jessica Yu wrote:

> So it doesn't look like a simple priority tweak will work, since we want a
> ftrace->klp->klp->ftrace ordering from module load->module unload. Perhaps we
> can split the ftrace and klp notifiers into separate coming and going handlers
> with the appropriate priorities set to enforce correct ordering (not terribly
> pretty, but it works). Is there a better solution?

Given the facts that:

- we want the ordering ftrace -> klp for load and klp -> ftrace for 
  unload

- we already do have separate klp_module_notify_coming() and 
  klp_module_notify_going(), and klp_module_notify() is just a stub that 
  decides which one to call

I'd suggest we keep just a single ftrace_module_notify() (with INT_MIN+1, 
priority), and then introduce separate notifiers

- klp_module_notify_going() with INT_MIN+2, that triggers action iff 
  action == MODULE_STATE_GOING

- klp_module_notify_going() with INT_MIN, that triggers action only iff 
  action == MODULE_STATE_COMING

Thanks,

-- 
Jiri Kosina
SUSE Labs