Re: [PATCH v2 33/44] ftrace: WARN on rcuidle

[Mark Rutland <mark.rutland@xxxxxxx>]

On Mon, Sep 19, 2022 at 12:00:12PM +0200, Peter Zijlstra wrote:
> CONFIG_GENERIC_ENTRY disallows any and all tracing when RCU isn't
> enabled.
> 
> XXX if s390 (the only other GENERIC_ENTRY user as of this writing)
> isn't comfortable with this, we could switch to
> HAVE_NOINSTR_VALIDATION which is x86_64 only atm.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  include/linux/tracepoint.h |   13 ++++++++++++-
>  kernel/trace/trace.c       |    3 +++
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -178,6 +178,16 @@ static inline struct tracepoint *tracepo
>  #endif /* CONFIG_HAVE_STATIC_CALL */
>  
>  /*
> + * CONFIG_GENERIC_ENTRY archs are expected to have sanitized entry and idle
> + * code that disallow any/all tracing/instrumentation when RCU isn't watching.
> + */
> +#ifdef CONFIG_GENERIC_ENTRY
> +#define RCUIDLE_COND(rcuidle)	(rcuidle)
> +#else
> +#define RCUIDLE_COND(rcuidle)	(rcuidle && in_nmi())
> +#endif

Could we make this depend on ARCH_WANTS_NO_INSTR instead?

That'll allow arm64 to check this even though we're not using the generic entry
code (and there's lots of work necessary to make that possible...).

Thanks,
Mark.

> +
> +/*
>   * it_func[0] is never NULL because there is at least one element in the array
>   * when the array itself is non NULL.
>   */
> @@ -189,7 +199,8 @@ static inline struct tracepoint *tracepo
>  			return;						\
>  									\
>  		/* srcu can't be used from NMI */			\
> -		WARN_ON_ONCE(rcuidle && in_nmi());			\
> +		if (WARN_ON_ONCE(RCUIDLE_COND(rcuidle)))		\
> +			return;						\
>  									\
>  		/* keep srcu and sched-rcu usage consistent */		\
>  		preempt_disable_notrace();				\
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3104,6 +3104,9 @@ void __trace_stack(struct trace_array *t
>  		return;
>  	}
>  
> +	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_GENERIC_ENTRY)))
> +		return;
> +
>  	/*
>  	 * When an NMI triggers, RCU is enabled via ct_nmi_enter(),
>  	 * but if the above rcu_is_watching() failed, then the NMI
> 
>