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Date: Mon, 3 Oct 2011 01:07:56 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>, linux-kernel@vger.kernel.org,
        Dipankar Sarma <dipankar@in.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Lai Jiangshan <laijs@cn.fujitsu.com>
Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833
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On Fri, Sep 30, 2011 at 12:24:38PM -0700, Paul E. McKenney wrote:
> +#ifdef CONFIG_PROVE_RCU
> +
> +/*
> + * Test whether the current CPU is idle.
> + */
> +int rcu_is_cpu_idle(void)
> +{
> +	return !rcu_dynticks_nesting;
> +}

Seems that's not used in the patch.

> +
> +#endif /* #ifdef CONFIG_PROVE_RCU */
<snip>
> +#ifdef CONFIG_PROVE_RCU
> +
>  /**
> - * rcu_irq_enter - inform RCU of entry to hard irq context
> + * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle
>   *
> - * If the CPU was idle with dynamic ticks active, this updates the
> - * rdtp->dynticks to let the RCU handling know that the CPU is active.
> + * If the current CPU is in its idle loop and is neither in an interrupt
> + * or NMI handler, return true.  The caller must have at least disabled
> + * preemption.
>   */
> -void rcu_irq_enter(void)
> +int rcu_is_cpu_idle(void)
>  {
> -	rcu_exit_nohz();
> +	return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
>  }

So that's not used in this patch but it's interesting for me
to backport "rcu: Detect illegal rcu dereference in extended quiescent state".

The above should be read from a preempt disabled section though
(remember "rcu: Fix preempt-unsafe debug check of rcu extended quiescent state")

Those functions should probably lay in a separate patch. But I don't mind
much keeping the things as is and use these APIs in my next patches though.
I'll just fix the preempt enabled thing above.