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Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Li Zefan <lizf@cn.fujitsu.com>, Steven Rostedt <rostedt@goodmis.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       LKML <linux-kernel@vger.kernel.org>
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Date: Mon, 24 Aug 2009 11:27:38 +0200
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On Mon, 2009-08-24 at 11:24 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > 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.
> 
> 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.