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

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Date:	Wed, 26 Aug 2009 15:31:06 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload

15:26, Peter Zijlstra wrote:
> 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?
> 

No, it won't fail. ;)

Instead, when foo is loaded, tracepoint_update_probe_range() will be
called, and the probe registered in bar will be added to the tracepoint.

Maybe we can do something in tracepoint_update_probe_range(). I'll try.
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