Re: [PATCH] tracing: Make tracepoint lockdep check actually test something

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 9 Mar 2023 16:56:03 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> A while ago where the trace events had the following:
> 
>    rcu_read_lock_sched_notrace();
>    rcu_dereference_sched(...);
>    rcu_read_unlock_sched_notrace();
> 
> If the tracepoint is enabled, it could trigger RCU issues if called in
> the wrong place. And this warning was only triggered if lockdep was
> enabled. If the tracepoint was never enabled with lockdep, the bug would
> not be caught. To handle this, the above sequence was done when lockdep
> was enabled regardless if the tracepoint was enabled or not (although the
> always enabled code really didn't do anything, it would still trigger a
> warning).
> 
> But a lot has changed since that lockdep code was added. One is, that
> sequence no longer triggers any warning. Another is, the tracepoint when
> enabled doesn't even do that sequence anymore.
> 
> The main check we care about today is whether RCU is "watching" or not.
> So if lockdep is enabled, always check if rcu_is_watching() which will
> trigger a warning if it is not (tracepoints require RCU to be watching).
> 
> Note, that old sequence did add a bit of overhead when lockdep was enabled,
> and with the latest kernel updates, would cause the system to slow down
> enough to trigger kernel "stalled" warnings.
> 
> Link: http://lore.kernel.org/lkml/20140806181801.GA4605@redhat.com
> Link: http://lore.kernel.org/lkml/20140807175204.C257CAC5@viggo.jf.intel.com
> Link: https://lore.kernel.org/lkml/20230307184645.521db5c9@gandalf.local.home/
> 

This check has been introduced by commit 3a630178fd5f ("tracing: generate RCU
warnings even when tracepoints are disabled"), and it also added a comment
above this macro.

 * When lockdep is enabled, we make sure to always do the RCU portions of
 * the tracepoint code, regardless of whether tracing is on. However,
 * don't check if the condition is false, due to interaction with idle
 * instrumentation. This lets us find RCU issues triggered with tracepoints
 * even when this tracepoint is off. This code has no purpose other than
 * poking RCU a bit.

I think at least the last sentence will be outdated by this fix.

Thank you,


> Cc: stable@vger.kernel.org
> Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  include/linux/tracepoint.h | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> 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),		\
> -- 
> 2.39.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>