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Date: Wed, 9 May 2007 22:41:13 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH 2/3] Add hard_irq_disable()
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Cc: linuxppc-dev@ozlabs.org, Rusty Russell <rusty@rustcorp.com.au>,
	linux-kernel@vger.kernel.org
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On Thu, 10 May 2007 15:25:58 +1000 Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> --- linux-cell.orig/include/linux/interrupt.h	2007-05-10 14:51:22.000000000 +1000
> +++ linux-cell/include/linux/interrupt.h	2007-05-10 15:18:04.000000000 +1000
> @@ -241,6 +241,16 @@ static inline void __deprecated save_and
>  #define save_and_cli(x)	save_and_cli(&x)
>  #endif /* CONFIG_SMP */
>  
> +/* Some architectures might implement lazy enabling/disabling of
> + * interrupts. In some cases, such as stop_machine, we might want
> + * to ensure that after a local_irq_disable(), interrupts have
> + * really been disabled in hardware. Such architectures need to
> + * implement the following hook.
> + */
> +#ifndef hard_irq_disable
> +#define hard_irq_disable()	do { } while(0)
> +#endif

We absolutely require that the architecture's hard_irq_disable() be defined
when we do this.  If it happens that the definition of hard_irq_disable()
is implemented three levels deep in nested includes then we risk getting
into a situation where different .c files see different implementations of
hard_irq_disable(), which could lead to confusing results, to say the least.

Your implementation comes via the inclusion of system.h which then includes
hw_irq.h.  That's perhaps a little fragile and it would be better to

a) include in the comment the name of the arch file which must implement
   hard_irq_disable() and

b) include that file directly from this one.

Oh, and your comment layout style is wrong ;)