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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 25, 2024 at 09:18:18AM -0500, Mathieu Desnoyers wrote:
> On 2024-11-23 12:38, Linus Torvalds wrote:

> 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)); \

So coding style would like braces here for it being multi-line. As
opposed to C that only mandates it for multi-statement. And then the
problem doesn't occur.

>                 }                                                       \
>                 if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {             \
>                         WARN_ONCE(!rcu_is_watching(),                   \
>                                   "RCU not watching for tracepoint");   \
>                 }                                                       \
>         }
> 

> 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")

Yeah,.. So strictly speaking the code is fine, but the various compilers
don't like it when that else dangles :/