linux-kernel - Re: [PATCH] tracing/profile: Fix profile_disable vs module

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Message-ID: <20090824092455.GA25267@elte.hu>
Date:	Mon, 24 Aug 2009 11:24:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload


* Peter Zijlstra <peterz@...radead.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@...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.

	Ingo
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