linux-kernel - Re: [PATCH 2/2] ftrace: Adjust priority of ftrace module notifier

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Message-ID: <20160129154505.GF4081@treble.redhat.com>
Date:	Fri, 29 Jan 2016 09:45:05 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jessica Yu <jeyu@...hat.com>, Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] ftrace: Adjust priority of ftrace module notifier

On Fri, Jan 29, 2016 at 09:38:00AM -0500, Steven Rostedt wrote:
> On Fri, 29 Jan 2016 01:43:47 -0500
> Jessica Yu <jeyu@...hat.com> wrote:
> 
> 
> > ---
> >  kernel/trace/ftrace.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index eca592f..bdd7bfc 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -5067,7 +5067,12 @@ static int ftrace_module_notify(struct notifier_block *self,
> >  
> >  struct notifier_block ftrace_module_nb = {
> >  	.notifier_call = ftrace_module_notify,
> > -	.priority = INT_MIN,	/* Run after anything that can remove kprobes */
> > +	/*
> > +	 * Run after anything that can remove kprobes and
> > +	 * after livepatch's going notifier, but run before
> > +	 * livepatch's coming notifier.
> > +	 */
> > +	.priority = INT_MIN+1,
> >  };
> >  
> >  void __init ftrace_init(void)
> 
> Actually, I rather break up the ftrace notifiers into two different
> notifiers. One for coming and one for going (I use to have that before
> hard coding the module updates in the module code).
> 
> Have the coming notifier be INT_MAX, where it runs before everything
> else (which it should, as tracing should be enabled then). And have the
> module going to stay INT_MIN to run after everything.

That sounds good to me.

If we do that then I still think it would be a good idea to split up the
livepatch notifiers, with:

- INT_MAX-1 for coming so that relocations are all written before any
  other notifiers (besides ftrace) get a chance to run.

- INT_MIN-1 for going.  I don't have a good specific reason, but I think
  the symmetry will create less surprises and possibly fewer bugs if the
  module's patched state as seen by the other notifiers is the same for
  coming and going.

-- 
Josh