linux-kernel - Re: [PATCH 08/16] tracing: have event_trace

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Message-ID: <alpine.DEB.2.00.0903122333110.18396@gandalf.stny.rr.com>
Date:	Thu, 12 Mar 2009 23:34:01 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 08/16] tracing: have event_trace_printk use static
 tracer


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

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