Messages in this thread | | | Date | Fri, 7 May 2010 11:30:35 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 2/9 - v2][RFC] tracing: Let tracepoints have data?passed to tracepoint callbacks |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > 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.
Yes, I am all for it.
As for the extra type checking, it is basically just trying to force you to generate matching caller-callee prototypes in your CPP macros. The goal is really to check that the data parameter type match in both the caller and callee. I see that as a mean to make sure nobody is going to try to take shortcuts by playing with the callback types in the "undefined behavior" zone of the C standard in future TRACE_EVENT() modifications.
Thanks,
Mathieu
> > -- Steve > >
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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