Re: [PATCH 08/16] tracing: have event_trace_printk use static tracer

Steven Rostedt
2009-03-13T03:34:26Z

On Fri, 13 Mar 2009, KOSAKI Motohiro wrote:

> > > > +#define event_trace_printk(ip, fmt, args...) \
> > > > +do { \
> > > > + __trace_printk_check_format(fmt, ##args); \
> > > > + tracing_record_cmdline(current); \
> > > > + if (__builtin_constant_p(fmt)) { \
> > > > + static const char *trace_printk_fmt \
> > > > + __attribute__((section("__trace_printk_fmt"))) = \
> > > > + __builtin_constant_p(fmt) ? fmt : NULL; \
> > >
> > > Why __builtin_constant_p(fmt) evaluate twice?
> >
> > It's explained in another patch. But this was a real PITA. We first tried
> > this with just the "if (__builtin_return_p(fmt))" but the way gcc works,
> > it handles the global data assignments before optimizing out condition
> > logic. Thus we ended up getting errors about can not initialize static
> > variable with a non constant.
> >
> > But the ? : operation of the assignment is optimized before the assignment
> > is made. Thus, if fmt is not constant, then we avoid this warning. Then
> > during the conditional optimization, gcc will remove that part of the code
> > altogether.
> >
> > Thus the double __builtin_constant_p(fmt) is needed twice. Try taking out
> > one of them and see what happens with:
> >
> > myfunc(const char *fmt) {
> >
> > event_trace_printk(fmt);
> >
> > }
> >
> > Of course the way this is made, we may not call it that way, but I wanted
> > to be safe.
>
> Thanks for kindful explain.
> So, I guess many developer feel it's strange.
> adding comment is better?

Yeah, I cut and pasted this out from the trace_printk in kernel.h where I
had the comment there. But I think you are right, I probably should copy
that comment here too.

-- Steve