linux-kernel - Re: [PATCH] printk: use this_cpu_{read|write} api on printk

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Date:	Fri, 26 Nov 2010 10:40:53 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, heiko.carstens@...ibm.com,
	tglx@...utronix.de, mingo@...e.hu
Subject: Re: [PATCH] printk: use this_cpu_{read|write} api on
 printk_pending

On Fri, 26 Nov 2010, Eric Dumazet wrote:

> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1074,17 +1074,17 @@ static DEFINE_PER_CPU(int, printk_pending);
>
>  void printk_tick(void)
>  {
> -	if (__get_cpu_var(printk_pending)) {
> -		__get_cpu_var(printk_pending) = 0;
> +	if (__this_cpu_read(printk_pending)) {
> +		__this_cpu_write(printk_pending, 0);
>  		wake_up_interruptible(&log_wait);
>  	}
>  }

The above looks ok.

>  int printk_needs_cpu(int cpu)
>  {
> -	if (unlikely(cpu_is_offline(cpu)))
> +	if (cpu_is_offline(cpu))
>  		printk_tick();
> -	return per_cpu(printk_pending, cpu);
> +	return __this_cpu_read(printk_pending);
>  }

This only works if printk_needs_cpu is always passed the current cpu. The
check for cpu_is_offline suggests that the function is also used for other
cpus. The __this_cpu_read() will then get the printk_pending stat for the
current cpu and not for the specified cpu.


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