To: linux-kernel@vger.kernel.org
From: Raja R Harinath <harinath@hurrynot.org>
Subject: Re: [PATCH for -tip 4/4] irq: for_each_irq_desc() makes simplify
Date: Thu, 01 Jan 2009 23:35:21 +0530
Message-ID: <87wsdezyu6.fsf@hariville.hurrynot.org>
References: <20081226121703.5CA3.KOSAKI.MOTOHIRO@jp.fujitsu.com>
	<20081226122830.5CAF.KOSAKI.MOTOHIRO@jp.fujitsu.com>
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Hi,

KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> writes:

> Subject: [PATCH] irq: for_each_irq_desc() makes simplify
> Impact: cleanup
>
> all for_each_irq_desc() usage point have !desc check.
> then its check can move into for_each_irq_desc() macro.
[snip]
> Index: b/include/linux/irqnr.h
> ===================================================================
> --- a/include/linux/irqnr.h
> +++ b/include/linux/irqnr.h
> @@ -25,10 +25,14 @@ extern struct irq_desc *irq_to_desc(unsi
>  
>  # define for_each_irq_desc(irq, desc)					\
>  	for (irq = 0, desc = irq_to_desc(irq); irq < nr_irqs;		\
> -	     irq++, desc = irq_to_desc(irq))
> +	     irq++, desc = irq_to_desc(irq))				\
> +		if (desc)
> +
> +
>  # define for_each_irq_desc_reverse(irq, desc)				\
>  	for (irq = nr_irqs - 1, desc = irq_to_desc(irq); irq >= 0;	\
> -	     irq--, desc = irq_to_desc(irq))
> +	     irq--, desc = irq_to_desc(irq))				\
> +		if (desc)

I know this has gone in, but isn't this naked 'if' unsafe.  Consider the
following hypothetical code:

  if (safe)
    for_each_irq_desc(irq, desc) {
       ...
    }
  else
    panic();

With the macro definition above, the loop would panic() each time !desc,
and _not_ panic() when !safe.  I'd consider this behaviour to be
unexpected, to say the least :-)

The fix is to change the

  if (desc)

in the macro to

  if (!desc) ; else

- Hari

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