Re: [PATCH v3 08/22] rcu,tracing: Create trace_rcu_{enter,exit}()

Steven Rostedt
2020-02-19T15:49:08Z

On Wed, 19 Feb 2020 15:47:32 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> To facilitate tracers that need RCU, add some helpers to wrap the
> magic required.
>
> The problem is that we can call into tracers (trace events and
> function tracing) while RCU isn't watching and this can happen from
> any context, including NMI.
>
> It is this latter that is causing most of the trouble; we must make
> sure in_nmi() returns true before we land in anything tracing,
> otherwise we cannot recover.
>
> These helpers are macros because of header-hell; they're placed here
> because of the proximity to nmi_{enter,exit{().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> include/linux/hardirq.h | 32 ++++++++++++++++++++++++++++++++

> +/*
> + * Tracing vs RCU
> + * --------------
> + *
> + * tracepoints and function-tracing can happen when RCU isn't watching (idle,
> + * or early IRQ/NMI entry).
> + *
> + * When it happens during idle or early during IRQ entry, tracing will have
> + * to inform RCU that it ought to pay attention, this is done by calling
> + * rcu_irq_enter_irqsave().
> + *
> + * On NMI entry, we must be very careful that tracing only happens after we've
> + * incremented preempt_count(), otherwise we cannot tell we're in NMI and take
> + * the special path.
> + */
> +
> +#define trace_rcu_enter() \
> +({ \
> + unsigned long state = 0; \
> + if (!rcu_is_watching()) { \
> + rcu_irq_enter_irqsave(); \
> + state = 1; \
> + } \
> + state; \
> +})
> +
> +#define trace_rcu_exit(state) \
> +do { \
> + if (state) \
> + rcu_irq_exit_irqsave(); \
> +} while (0)
> +

Is there a reason that these can't be static __always_inline functions?

-- Steve

> #endif /* LINUX_HARDIRQ_H */
>