Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Sep 05, 2013 at 12:52:34PM -0700, Paul E. McKenney wrote:
> 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,
> 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.
> 
> 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>
> 
>  include/linux/rcupdate.h |    1 +
>  kernel/rcutree.c         |   12 ++++++++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 15d33d9..1c7112c 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -225,6 +225,7 @@ extern void rcu_idle_enter(void);
>  extern void rcu_idle_exit(void);
>  extern void rcu_irq_enter(void);
>  extern void rcu_irq_exit(void);
> +extern bool rcu_watching_this_cpu(void);
>  
>  #ifdef CONFIG_RCU_USER_QS
>  extern void rcu_user_enter(void);
> 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);
> +}

There is also rcu_is_cpu_idle().

Thanks.

> +
> +/**
>   * 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)
>