Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()

Joel Fernandes
2020-02-12T23:20:07Z

On Wed, Feb 12, 2020 at 10:01:42PM +0100, Peter Zijlstra 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 | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 48 insertions(+)
>
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -89,4 +89,52 @@ extern void irq_exit(void);
> arch_nmi_exit(); \
> } while (0)
>
> +/*
> + * 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 __TR_IRQ 1
> +#define __TR_NMI 2
> +
> +#define trace_rcu_enter() \
> +({ \
> + unsigned long state = 0; \
> + if (!rcu_is_watching()) { \
> + if (in_nmi()) { \
> + state = __TR_NMI; \
> + rcu_nmi_enter(); \
> + } else { \
> + state = __TR_IRQ; \
> + rcu_irq_enter_irqsave(); \

I think this can be simplified. You don't need to rely on in_nmi() here. I
believe for NMI's, you can just call rcu_irq_enter_irqsave() and that should
be sufficient to get RCU watching. Paul can correct me if I'm wrong, but I am
pretty sure that would work.

In fact, I think a better naming for rcu_irq_enter_irqsave() pair could be
(in the first patch):

rcu_ensure_watching_begin();
rcu_ensure_watching_end();

thanks,

- Joel

> + } \
> + } \
> + state; \
> +})
> +
> +#define trace_rcu_exit(state) \
> +do { \
> + switch (state) { \
> + case __TR_IRQ: \
> + rcu_irq_exit_irqsave(); \
> + break; \
> + case __TR_NMI: \
> + rcu_nmi_exit(); \
> + break; \
> + default: \
> + break; \
> + } \
> +} while (0)
> +
> #endif /* LINUX_HARDIRQ_H */
>
>