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Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Steven Rostedt <rostedt@goodmis.org>, Li Zefan <lizf@cn.fujitsu.com>,
       Ingo Molnar <mingo@elte.hu>, Frederic Weisbecker <fweisbec@gmail.com>,
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Date: Wed, 26 Aug 2009 21:14:06 +0200
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On Wed, 2009-08-26 at 12:46 -0400, Mathieu Desnoyers wrote:
> Registering tracepoints even when no tracepoint definition is currently
> visible is the intended allowed behavior. Let's say we need to trace
> something happening in module init: if we disallow registering the tp
> callback before the module is initialized, we run in a chicken and egg
> problem.
> 
> So I am trying to figure out the problem source there. Is it that
> modules containing the tp callbacks need to know if those are actually
> connected to an instrumented module ? Or is it that the instrumented
> module needs to know if a probe module is connected to is ? Or is it the
> teardown of the probe module ? No refcount is needed there, because we
> surround the probe call by preempt disable/enable, and we use
> synchronize_sched() before removing the module which contains probe
> callbacks.
> 
> Mathieu-trying-to-figure-out-what-this-whole-thread-is-about :)

OK, so the whole point seems to be that tracepoints have the funny thing
you describe above, whereas the things ftrace makes out of TRACE_EVENT()
get instantiated along with modules.

The reason why I rejected the initial patch (and I still think that that
fix is at the wrong layer) is that I, as a consumer of whatever
TRACE_EVENT() offers, should never need to consider modules.