Messages in this thread | | | Date | Fri, 30 Apr 2010 17:02:22 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 04/10][RFC] tracing: Remove per event trace registering |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > On Fri, 2010-04-30 at 16:07 -0400, Mathieu Desnoyers wrote: > > * Steven Rostedt (rostedt@goodmis.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. > > > > OK, so you prefer that, I don't, but I also don't care, so I could > easily change it. > > > > > 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. > > Fine, but I hardly see it as obfuscation. But my question again, even if > we do change this. What is this test testing? To me, it is checking that > CPP works.
It's checking that the macros generated compatible call/callback prototypes, yes. It comes down to using the compiler type-checking to double-check that the macros are fine.
Thanks,
Mathieu
> > -- Steve > >
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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