Re: [PATCH] trace_printk: replace _______STR with __UNIQUE_ID(STR)

[David Laight <david.laight.linux@gmail.com>]

On Wed, 29 Apr 2026 13:42:26 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 30 Apr 2026 00:57:07 +0800
> Qian-Yu Lin <tiffany019230@gmail.com> wrote:
> 
> > The macro trace_printk() uses a hardcoded identifier _______STR
> > within a statement expression, which can lead to variable name
> > shadowing if a caller happens to use the same name in its scope.  
> 
> Has this ever been a problem?
> 
> > 
> > Following the pattern in commit 24ba53017e18 ("rcu: Replace ________p1
> > and _________p1 with __UNIQUE_ID(rcu)") and commit 589a9785ee3a
> >  ("min/max: remove sparse warnings when they're nested"), replace the
> >  hardcoded identifier with __UNIQUE_ID(STR).

min and max do get nested - so it is important that the 'local'
variables have different names - otherwise you can get invalid expansions.

No one is going to have: trace_printk(fmt1, trace_printk(ftm2, ...), ...)
it just doesn't make sense.

There is a slight problem the ____________________________STR might
be used by an entirely different #define.
Just using _trace_printk_str[] would make this pretty unlikely.
It would also have the advantage of making the .i file a bit easier
to read, all the UNIQUE names in min/max output make it really hard
to see what the output actually means.

> > 
> > Since __UNIQUE_ID() must be expanded once to remain consistent across
> > declaration and sizeof() within the statement expression, introduce a
> > nested helper macro ___trace_printk.  
> 
> Hmm, so we are replacing one name with underscores with another name
> with underscores?
> 
> > 
> > Signed-off-by: Qian-Yu Lin <tiffany019230@gmail.com>
> > ---
> >  include/linux/trace_printk.h | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/trace_printk.h b/include/linux/trace_printk.h
> > index 2670ec7f4262..060eccb40838 100644
> > --- a/include/linux/trace_printk.h
> > +++ b/include/linux/trace_printk.h
> > @@ -2,6 +2,7 @@
> >  #ifndef _LINUX_TRACE_PRINTK_H
> >  #define _LINUX_TRACE_PRINTK_H
> >  
> > +#include <linux/compiler.h>  
> 
> People are already saying that trace_printk.h slows down the compile.
> Does this add any overhead to the compile?

A little - nothing is free.

	David

> 
> -- Steve
> 
> 
> >  #include <linux/compiler_attributes.h>
> >  #include <linux/instruction_pointer.h>
> >  #include <linux/stddef.h>
> > @@ -84,15 +85,18 @@ do {									\
> >   * let gcc optimize the rest.
> >   */
> >  
> > -#define trace_printk(fmt, ...)				\
> > +#define ___trace_printk(fmt, str, ...)				\
> >  do {							\
> > -	char _______STR[] = __stringify((__VA_ARGS__));	\
> > -	if (sizeof(_______STR) > 3)			\
> > +	char str[] = __stringify((__VA_ARGS__));	\
> > +	if (sizeof(str) > 3)			\
> >  		do_trace_printk(fmt, ##__VA_ARGS__);	\
> >  	else						\
> >  		trace_puts(fmt);			\
> >  } while (0)
> >  
> > +#define trace_printk(fmt, ...) \
> > +	___trace_printk(fmt, __UNIQUE_ID(str), ##__VA_ARGS__)
> > +
> >  #define do_trace_printk(fmt, args...)					\
> >  do {									\
> >  	static const char *trace_printk_fmt __used			\  
> 
>