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

[Przemek Kitszel <przemyslaw.kitszel@intel.com>]

On 11/25/24 15:26, Peter Zijlstra wrote:
> 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 :/

At one point I had a version that did:
	if (0)
label: ;
	else
		for (....)

but it is goto-jumping back in the code
https://lore.kernel.org/netdev/20241001145718.8962-1-przemyslaw.kitszel@intel.com/#t

I could switch to it again to reduce noise like this problem, but such
change would be to essentially allow bad formatting