From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S1754579AbZHYGWB@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754579AbZHYGWB (ORCPT <rfc822;w@1wt.eu>);
	Tue, 25 Aug 2009 02:22:01 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754295AbZHYGV7
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Tue, 25 Aug 2009 02:21:59 -0400
Received: from casper.infradead.org ([85.118.1.10]:57887 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754271AbZHYGV6 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 25 Aug 2009 02:21:58 -0400
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload
From: Peter Zijlstra <peterz@infradead.org>
To: Li Zefan <lizf@cn.fujitsu.com>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <4A937505.5000209@cn.fujitsu.com>
References: <4A9214E3.2070807@cn.fujitsu.com>
	 <1251093660.7538.119.camel@twins> <4A923197.4040708@cn.fujitsu.com>
	 <1251097012.7538.123.camel@twins>  <20090824092455.GA25267@elte.hu>
	 <1251106058.7538.149.camel@twins>  <4A937505.5000209@cn.fujitsu.com>
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
Date: Tue, 25 Aug 2009 08:21:06 +0200
Message-Id: <1251181266.7538.1016.camel@twins>
Mime-Version: 1.0
X-Mailer: Evolution 2.26.1 
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, 2009-08-25 at 13:22 +0800, Li Zefan wrote:
> >>>>>> If the correspoding module is unloaded before ftrace_profile_disable()
> >>>>>> is called, event->profile_disable() won't be called, which can
> >>>>>> cause oops:
> >>>>>>
> >>>>>>   # insmod trace-events-sample.ko
> >>>>>>   # perf record -f -a -e sample:foo_bar sleep 3 &
> >>>>>>   # sleep 1
> >>>>>>   # rmmod trace_events_sample
> >>>>>>   # insmod trace-events-sample.ko
> >>>>>>   OOPS!
> >>>>>>
> >>>>>> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> >>>>>
> >>>>> Hrmm, feel fragile, why don't we check if all a modules tracepoints are
> >>>>> unused on unload?
> >>>>>
> >>>> I don't think it's fragile. We are profiling via a module's
> >>>> tracepoint, so we should pin the module, via module_get().
> >>>> If event->profile_enable() has been calld, we should make
> >>>> sure it's profile_disable() will be called.
> >>> What I call fragile is that everyone registering a tracepoint 
> >>> callback will now apparently need to worry about modules, _that_ 
> >>> is fragile.
> >>>
> >>> Either make module unload look at tracepoint users, or place the 
> >>> try_get_module() in the registration hooks so that regular users 
> >>> don't need to worry about it.
> >> The bug found by Li needs to be fixed obviously.
> >>
> >> I tend to agree with you that this does not appear to be the best 
> >> place to do it: so you suggest to implicitly increase the module 
> >> refcount on callback registr instead? (and releasing it when 
> >> unregistering)
> >>
> >> Same end result, slightly cleaner place to bump the refcount.
> > 
> > Yes, because the user of tracepoints should never need to care about
> > modules.
> > 
> 
> I'm afraid it is not feasible to bump module refcnt implicitly
> in tracepoint_probe_register().
> 
> If a tracepoint is registered in module_init, and unregistered
> in module_exit (see sample/tracepoints), the module is unloadable:
> 
>  insmod
>  ->call mod->init()
>    ->trace_reg_foo()
>      ->module_get()
> 
>  rmmod
>  ->check mod refcnt
>  ->call mod->exit()
>    ->trace_unreg_foo()
>      ->module_put()

Not tracepoint_probe_{un,}register(), in {un,}register_trace_$call().

Basically avoid module unload when a tracepoint from that module has
registered callbacks.