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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>
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	 <1251093660.7538.119.camel@twins>  <4A923197.4040708@cn.fujitsu.com>
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Date: Mon, 24 Aug 2009 08:56:52 +0200
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On Mon, 2009-08-24 at 14:22 +0800, Li Zefan wrote:
> Peter Zijlstra wrote:
> > On Mon, 2009-08-24 at 12:19 +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.

But this is rediculous.