Date: Thu, 5 Sep 2013 19:40:05 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: paulmck@linux.vnet.ibm.com
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com,
        tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com,
        edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com,
        sbw@mit.edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?
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On Thu, 5 Sep 2013 14:05:37 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:


> 
> rcu: Is it safe to enter an RCU read-side critical section?
> 
> There is currently no way for kernel code to determine whether it
> is safe to enter an RCU read-side critical section, in other words,

Shouldn't that be a semi-colon?

	"read-side critical section; in other words,"

> whether or not RCU is paying attention to the currently running CPU.
> Given the large and increasing quantity of code shared by the idle loop
> and non-idle code, the this shortcoming is becoming increasingly painful.

s/the//

> 
> This commit therefore adds rcu_watching_this_cpu(), which returns true
> if it is safe to enter an RCU read-side critical section on the currently
> running CPU.  This function is quite fast, using only a __this_cpu_read().
> However, the caller must disable preemption.
> 
> Reported-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 15d33d9..7c024fd 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -225,6 +225,9 @@ extern void rcu_idle_enter(void);
>  extern void rcu_idle_exit(void);
>  extern void rcu_irq_enter(void);
>  extern void rcu_irq_exit(void);
> +#if defined(CONFIG_SMP) || defined(CONFIG_RCU_TRACE)
> +extern bool rcu_watching_this_cpu(void);

I'm assuming that rcu is always watching a CPU if "!SMP" and
"!RCU_TRACE". Thus you still need to have:

#else
static inline bool rcu_watching_this_cpu(void)
{
	return true;
}

Otherwise we still fail to compile.

-- Steve

> +#endif /* #if defined(CONFIG_SMP) || defined(CONFIG_RCU_TRACE) */
>  
>  #ifdef CONFIG_RCU_USER_QS
>  extern void rcu_user_enter(void);
> diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> index 7e3b0d6..fce820f 100644
> --- a/kernel/rcutiny.c
> +++ b/kernel/rcutiny.c
> @@ -189,6 +189,17 @@ EXPORT_SYMBOL(rcu_is_cpu_idle);
>  
>  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
>  
> +#ifdef CONFIG_RCU_TRACE
> +/*
> + * Test whether the current CPU can safely enter RCU read-side critical
> + * sections.  The caller must at least have disabled interrupts.
> + */
> +bool rcu_watching_this_cpu(void)
> +{
> +	return !!rcu_dynticks_nesting;
> +}
> +#endif /* #ifdef CONFIG_RCU_TRACE */
> +
>  /*
>   * Test whether the current CPU was interrupted from idle.  Nested
>   * interrupts don't count, we must be running at the first interrupt
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index a06d172..7b8fcee 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -710,6 +710,18 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
>  #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
>  
>  /**
> + * rcu_watching_this_cpu - are RCU read-side critical sections safe?
> + *
> + * Return true if RCU is watching the running CPU, which means that this
> + * CPU can safely enter RCU read-side critical sections.  The caller must
> + * have at least disabled preemption.
> + */
> +bool rcu_watching_this_cpu(void)
> +{
> +	return !!__this_cpu_read(rcu_dynticks.dynticks_nesting);
> +}
> +
> +/**
>   * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle
>   *
>   * If the current CPU is idle or running at a first-level (not nested)

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