linux-kernel - Re: [RFC PATCH 4/5] tracing: Remove conditional locking from __DO

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Message-ID: <20241125142606.GG38837@noisy.programming.kicks-ass.net>
Date: Mon, 25 Nov 2024 15:26:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Jeanson <mjeanson@...icios.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>, Yonghong Song <yhs@...com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andrii Nakryiko <andrii.nakryiko@...il.com>, bpf@...r.kernel.org,
	Joel Fernandes <joel@...lfernandes.org>,
	Jordan Rife <jrife@...gle.com>, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/5] tracing: Remove conditional locking from
 __DO_TRACE()

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 :/