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

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Date:	Wed, 26 Aug 2009 09:39:45 +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>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload

On Wed, 2009-08-26 at 15:31 +0800, Li Zefan wrote:
> 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.

*blink*

so we'll succeed in registering a tracepoint we know isn't there?
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