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

[Qian-Yu Lin]

On Wed, Apr 29, 2026 at 01:42:26PM -0400, Steven Rostedt wrote:
> On Thu, 30 Apr 2026 00:57:07 +0800
> Qian-Yu Lin <tiffany019230@xxxxxxxxx> 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?
> 

No, this has not been a practical problem. However, the long-underscore
name ___STR is an indication of a potential shadowing risk.

> > 
> > 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).
> > 
> > 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@xxxxxxxxx>
> > ---
> >  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?
> 
> -- Steve

Yes. I measured compile time of kernel/trace/ring_buffer_benchmark.o
after make clean on an x86_64 machine running Ubuntu 24.04 LTS:

  - Original _______STR:                 49.8s
  - v1 with __UNIQUE_ID (compiler.h):    53.5s
  - compound literal (no extra include): 33.2s

I propose using a compound literal in v2, which eliminates the local
variable entirely and requires no extra include:

#define trace_printk(fmt, ...)                          \
do {                                                    \
    if (sizeof((char[])                             \
        {__stringify((__VA_ARGS__))}) > 3)      \
        do_trace_printk(fmt, ##__VA_ARGS__);    \
    else                                            \
        trace_puts(fmt);                        \
} while (0)

This fully eliminates the shadowing risk without any compile overhead.

Qian-Yu

> 
> 
> >  #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			\
>