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

Steven Rostedt
2022-09-26T18:41:57Z

Nit, the subject should have "tracing:" an not "ftrace:" as the former
encompasses the tracing infrastructure and the latter is for the function
hook part of that.

On Mon, 19 Sep 2022 12:00:12 +0200
Peter Zijlstra <peterz@infradead.org> 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@infradead.org>
> ---
> 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

Should probably move the below comment to here:

/* srcu can't be used from NMI */

> +#define RCUIDLE_COND(rcuidle) (rcuidle && in_nmi())
> +#endif
> +
> +/*
> * 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 */ \

And remove the above.

-- Steve

> - 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
>