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Date: Wed, 28 Feb 2024 10:48:32 -0800
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] tracepoints: Use WARN() and not WARN_ON() for warnings
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On Wed, Feb 28, 2024 at 01:31:12PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> There are two WARN_ON*() warnings in tracepoint.h that deal with RCU
> usage. But when they trigger, especially from using a TRACE_EVENT()
> macro, the information is not very helpful and is confusing:
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 0 PID: 0 at include/trace/events/lock.h:24 lock_acquire+0x2b2/0x2d0
> 
> Where the above warning takes you to:
> 
>  TRACE_EVENT(lock_acquire,  <<<--- line 24 in lock.h
> 
> 	TP_PROTO(struct lockdep_map *lock, unsigned int subclass,
> 		int trylock, int read, int check,
> 		struct lockdep_map *next_lock, unsigned long ip),
> 	[..]
> 
> Change the WARN_ON_ONCE() to WARN_ONCE() and add a string that allows
> someone to search for exactly where the bug happened.
> 
> Reported-by: Borislav Petkov <bp@alien8.de>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  include/linux/tracepoint.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 88c0ba623ee6..689b6d71590e 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -199,7 +199,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		if (!(cond))						\
>  			return;						\
>  									\
> -		if (WARN_ON_ONCE(RCUIDLE_COND(rcuidle)))		\
> +		if (WARN_ONCE(RCUIDLE_COND(rcuidle),			\
> +			      "Bad RCU usage for tracepoint"))		\
>  			return;						\
>  									\
>  		/* keep srcu and sched-rcu usage consistent */		\
> @@ -259,7 +260,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  				TP_ARGS(args),				\
>  				TP_CONDITION(cond), 0);			\
>  		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
> -			WARN_ON_ONCE(!rcu_is_watching());		\
> +			WARN_ONCE(!rcu_is_watching(),			\
> +				  "RCU not watching for tracepoint");	\
>  		}							\
>  	}								\
>  	__DECLARE_TRACE_RCU(name, PARAMS(proto), PARAMS(args),		\
> -- 
> 2.43.0
>