linux-kernel - Re: [PATCH v3 08/22] rcu,tracing: Create trace_rcu

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Date:   Wed, 19 Feb 2020 10:49:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v3 08/22] rcu,tracing: Create trace_rcu_{enter,exit}()

On Wed, 19 Feb 2020 15:47:32 +0100
Peter Zijlstra <peterz@...radead.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@...radead.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 */
>