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 <[email protected]> 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 <[email protected]>
> > ---
> > 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 \
>
