Re: [RFC PATCH 4/5] tracing: Remove conditional locking from __DO_TRACE()

[Mathieu Desnoyers]

On 2024-11-23 12:38, Linus Torvalds wrote:
> On Sat, 23 Nov 2024 at 07:31, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> >   include/linux/tracepoint.h | 45 ++++++++++----------------------------
> >   1 file changed, 12 insertions(+), 33 deletions(-)
>
> Thanks. This looks much more straightforward, and obviously is smaller too.
>
> Side note: I realize I was the one suggesting "scoped_guard()", but
> looking at the patch I do think that just unnecessarily added another
> level of indentation. Since you already wrote the
>
>      if (cond) {
>          ..
>      }
>
> part as a block statement, there's no upside to the guard having its
> own scoped block, so instead of
>
>      if (cond) { \
>          scoped_guard(preempt_notrace)           \
>              __DO_TRACE_CALL(name, TP_ARGS(args)); \
>      }
>
> this might be simpler as just a plain "guard()" and one less indentation:
>
>      if (cond) { \
>          guard(preempt_notrace);           \
>          __DO_TRACE_CALL(name, TP_ARGS(args)); \
>      }
>
> but by now this is just an unimportant detail.
>
> I think I suggested scoped_guard() mainly because that would then just
> make the "{ }" in the if-statement superfluous, but that's such a
> random reason that it *really* doesn't matter.

I tried the following alteration to the code, which triggers an
unexpected compiler warning on master, but not on v6.12. I suspect
this is something worth discussing:

        static inline void trace_##name(proto)                          \
        {                                                               \
                if (static_branch_unlikely(&__tracepoint_##name.key)) { \
                        if (cond)                                       \
                                scoped_guard(preempt_notrace)           \
                                        __DO_TRACE_CALL(name, TP_ARGS(args)); \
                }                                                       \
                if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {             \
                        WARN_ONCE(!rcu_is_watching(),                   \
                                  "RCU not watching for tracepoint");   \
                }                                                       \
        }

It triggers this warning with gcc version 12.2.0 (Debian 12.2.0-14):

In file included from ./include/trace/syscall.h:5,
                 from ./include/linux/syscalls.h:94,
                 from init/main.c:21:
./include/trace/events/tlb.h: In function ‘trace_tlb_flush’:
./include/linux/tracepoint.h:261:28: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wdangling-else]
  261 |                         if (cond)                                       \
      |                            ^
./include/linux/tracepoint.h:446:9: note: in expansion of macro ‘__DECLARE_TRACE’
  446 |         __DECLARE_TRACE(name, PARAMS(proto), PARAMS(args),              \
      |         ^~~~~~~~~~~~~~~
./include/linux/tracepoint.h:584:9: note: in expansion of macro ‘DECLARE_TRACE’
  584 |         DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))
      |         ^~~~~~~~~~~~~
./include/trace/events/tlb.h:38:1: note: in expansion of macro ‘TRACE_EVENT’
   38 | TRACE_EVENT(tlb_flush,
      | ^~~~~~~~~~~

I suspect this is caused by the "else" at the end of the __scoped_guard() macro:

#define __scoped_guard(_name, _label, args...)                          \
        for (CLASS(_name, scope)(args);                                 \
             __guard_ptr(_name)(&scope) || !__is_cond_ptr(_name);       \
             ({ goto _label; }))                                        \
                if (0) {                                                \
_label:                                                                 \
                        break;                                          \
                } else

#define scoped_guard(_name, args...)    \
        __scoped_guard(_name, __UNIQUE_ID(label), args)

AFAIU this is a new warning introduced by

commit fcc22ac5baf ("cleanup: Adjust scoped_guard() macros to avoid potential warning")

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com