linux-kernel - Re: [PATCH] tracing, hardirq: Do not test lockdep on irq trace points when disabled

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Message-ID: <20230308115606.0b6a193c@gandalf.local.home>
Date:   Wed, 8 Mar 2023 11:56:06 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Ross Zwisler <zwisler@...gle.com>
Subject: Re: [PATCH] tracing, hardirq: Do not test lockdep on irq trace
 points when disabled

On Wed, 8 Mar 2023 15:39:25 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> > 		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
> > 			rcu_read_lock_sched_notrace();			\
> > 			rcu_dereference_sched(__tracepoint_##name.funcs);\
> > 			rcu_read_unlock_sched_notrace();		\
> > 		}							\
> > 	}								\  
> 
> > Where it will test lockdep for trace points even when they are not
> > enabled, to make sure they do not cause RCU issues, and lockdep will
> > trigger even when the trace points are not enabled.  
> 
> I'm confused what that's actually trying to do..
> 
> You're not tickling the rcu_is_watching() check, because
> rcu_read_lock_sched_notrace() doesn't have that. You're not tickling
> RCU_LOCKDEP_WARN() because you did rcu_read_lock_sched_notrace().
> 
> So what?!?

Actually, I think the proper changes is to just add "rcu_is_watching()"
warning here?

That code is from 2014 where it simulated what was done in DO_TRACE() and I
think back then, those calls did trigger warnings. But this code has not
kept up with the changes in DO_TRACE.

So something like this instead?

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index e299f29375bb..d3a221158ab1 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -260,9 +260,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 				TP_ARGS(args),				\
 				TP_CONDITION(cond), 0);			\
 		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
-			rcu_read_lock_sched_notrace();			\
-			rcu_dereference_sched(__tracepoint_##name.funcs);\
-			rcu_read_unlock_sched_notrace();		\
+			WARN_ON_ONCE(!rcu_is_watching());		\
 		}							\
 	}								\
 	__DECLARE_TRACE_RCU(name, PARAMS(proto), PARAMS(args),		\


-- Steve