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SubjectRe: [PATCH 2/9 - v2][RFC] tracing: Let tracepoints have data?passed to tracepoint callbacks
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On Fri, 2010-05-07 at 11:08 -0400, Mathieu Desnoyers wrote:

> > > Can you show me where the C standard says it is safe to do so ?
> >
> > No, but it seems safe in the kernel ;-)
>
> The use of "seems" here does not give me a warm feeling of safety. ;)

Right, which is why I added the below.

>
> >
> > But that said. There is another option that will conform to this, and
> > that is to add flags to registering tracepoints. I already wrote a patch
> > for this in trying to do some other work (that I threw away).
> >
> >
> > So here's the proposal.
> >
> > Change struct tracepoint_func to...
> >
> > struct tracepoint_func {
> > void *func;
> > void *data;
> > unsigned int flags;
> > };
> >
> >
> > The flags is set when registered. If a function is registered with data,
> > then the flags field will be set. Then the calling of the function can
> > be:
> >
> > if ((it_func_ptr)->flags & TP_FL_DATA)
> > ((void(*)(proto, void *))(it_func)(args, __data);
> > else
> > ((void(*)(proto))(it_func)(args);
> >
> > This would comply with the C standard.
>
> This would also add a branch on the tracing fast path, which I would like to
> avoid. Why can't we simply change all prototypes to take an extra void *__data
> parameter instead ?

I'm fine with making the data parameter mandatory with all tracers. Thus
the call back must require it. I would then move the data parameter from
the end to the beginning.

So a tracepoint with proto, will have a callback:

void callback(void *data, proto);

I'm fine with forcing all callbacks to include a data parameter if you
are. This would also make the changes simpler.

-- Steve




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