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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>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
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Date: Wed, 26 Aug 2009 09:26:28 +0200
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On Wed, 2009-08-26 at 15:10 +0800, Li Zefan wrote:
> Peter Zijlstra wrote:
> > On Wed, 2009-08-26 at 08:46 +0200, Peter Zijlstra wrote:
> > 
> >> Aahh, I see the bug, its only ftrace that knows about the module, not
> >> tracepoints themselves, _that_ needs fixing.
> > 
> > You could possibly do something like:
> > 
> >  struct module *tp_mod = __module_address(&some_tp_symbol);
> >  struct module *cb_mod = __module_text_address(func);
> > 
> >  if (tp_mod && tp_mod != cb_mod) {
> > 	ret = try_get_module(tp_mod);
> > 	if (ret)
> > 		goto fail;
> >  }
> > 
> > in register_trace_##name() or thereabout.
> > 
> 
> Actually I tried it, but it didn't work. As I said, You can't find
> any tp symbol when registering tp callback. The same example again:
> 
> 	In module bar, we have register_trace_foo()
> 	In module foo, we have DEFINE_TRACE(foo) and trace_foo().
> 
> bar doesn't know any symbol of foo, so it can't bump foo's refcnt,

Well, clearly it knows about register_trace_foo() which itself knows at
least one symbol that should be in module foo, right? How else could it
register a callback in that module (if it were loaded)?

It appears to use some intermediate code, in which case the intermediate
code knows about foo, which too solves our problem.

> *Note: you can load module bar without loading module foo*

In which case the tracepoint registration fails, right?