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

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Date:	Tue, 25 Aug 2009 08:21:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, 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

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@...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.
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