Date:	Fri, 30 Apr 2010 16:07:46 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 04/10][RFC] tracing: Remove per event trace registering

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Fri, 2010-04-30 at 15:06 -0400, Mathieu Desnoyers wrote:
> 
> > > If it is possible sure, but that's the point. Where do you add the
> > > check? The typecast is in the C code that is constant for all trace
> > > events.
> > 
> > You can add the call to the static inline type check directly within the
> > generated probe function, right after the local variable declarations.
> 
> Well, one thing, the callback is not going to be the same as the
> DECLARE_TRACE() because the prototype ends with "void *data", and the
> function being called actually uses the type of that data.
> 
> We now will have:
> 
> 	DEFINE_TRACE(mytracepoint, int myarg, myarg);
> 
> 	void mycallback(int myarg, struct mystuct *mydata);
> 
> 	register_trace_mytracepoint_data(mycallback, mydata)
> 
> There's no place in DEFINE_TRACE to be able to test the type of data
> that is being passed back. I could make the calling function be:
> 
> 	void mycallback(int myarg, void *data)
> 	{
> 		struct mystruct *mydata = data;
> 	[...]
> 
> Because the data is defined uniquely by the caller that registers a
> callback. Each function can register its own data type.

Yep. There would need to be a cast from void * to struct mystruct *
at the beginning of the callback as you propose here. I prefer this cast
to be explicit (as proposed here) rather than hidden within the entire
function call (void *) cast.

> 
> > 
> > > 
> > > > 
> > > > I also don't trust that these complex TRACE_EVENT() preprocessor macros
> > > 
> > > Thanks for your vote of confidence.
> > 
> > Please don't take this personally. As I said above, I include myself in
> > the list of people I don't trust to write entirely bug-free code. I'm
> > just saying that we should not overlook a possibility to detect more
> > bugs automatically when we have one, especially if this results in no
> > object code change.
> 
> The point being is that this is not about buggy code, but the fact that
> the same data is being used in two places, you want to test to make sure
> it is the same. I don't see how this helps.

See my comment above about specifically casting the void *data parameter
rather than relying on casting of the whole callback function pointer
type to void *.

> 
> 
> 
> > 
> > > 
> > > > will never ever have bugs. That's just doomed to happen one day or
> > > > another. Again, call me paranoid if you like, but I think adding this
> > > > type checking is justified.
> > > 
> > > Where do you add the typecheck?? As I said before, if the TRACE_EVENT()
> > > macros are broken, then so will the typecheck, and it will not catch the
> > > errors.
> > > 
> > > Sure the event macros can have bugs, but if it does then it will have
> > > bugs for all. Because it is automated. If there is a bug, it wont be
> > > because of a missed type being passed in, it would be because of one of
> > > the extra macros we have that processes the same type incorrectly.
> > > 
> > > > 
> > > > I am providing the type check implementation in a separate email. It
> > > > will need to be extended to support the extra data parameter you plan to
> > > > add.
> > > 
> > > I saw the patch, but how does it help?
> > > 
> > > I use "proto" to make the tracepoint and the callback, so I can add
> > > somewhere this "check_trace_callback_type_##name(proto)", but if the
> > > macros break somehow, that means proto changed between two references of
> > > it, but what keeps proto from breaking at both callback creation and the
> > > typecheck.
> > > 
> > > Basically, you are saying that somehow the argument "proto" can change
> > > between two uses of it. I don't really see that happening, and I'm not
> > > paranoid enough to think that's an issue. Adding checks that don't
> > > really check anything, honestly I find a waste, and just more confusion
> > > in the macros.
> > 
> > In the TRACE_EVENT() case, without the extra "void *data" argument,
> > it is indeed checking that the "proto" of the callback you create is
> > that same as the "proto" expected by the tracepoint call. However, given
> > that you plan on adding other parameters besides "proto", then the added
> > type-checking makes more and more sense.
> 
> But you can not test it! That's my point.
> 
> The first part of proto will be the same, and that's all we can test.
> But the data parameter that the DECLARE_TRACE() is going to create will
> be void *. Which means we can't test it. This is something that C lacks,
> and we could test it in C++ if we did this with templates. The only way
> to test it is at runtime with a magic number in the data field.
> 
> This is the same as the file->private data. You can't test it at build
> time.
> 
> Let me explain this again:
> 
> 	DECLARE_TRACE(name, proto, args);
> 
> Will call the function like:
> 
> 	callback(args, data);
> 
> The callback will be at best:
> 
> 	int callback(proto, void *data);
> 
> 
> because the data being passed in is not defined yet. It is defined at
> the point of the registering of the callback. You can have two callbacks
> registered to the same tracepoint with two different types as the data
> field.
> 
> So what is it that this check is testing?

It's making sure that TRACE_EVENT() creates callbacks with the following
signature:

  void callback(proto, void *data)

rather than

  void callback(proto, struct somestruct *data)

and forces the cast to be done within the callback rather than casting
the whole function pointer type to void *, assuming types to match. I
prefer to leave the cast outside of the tracepoint infrastructure, so we
do not obfuscate the fact that an explicit type cast is needed there.

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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