Subject: Re: [PATCH] synchronize_irq needs a barrier

From: Andrew Morton <[email-masked]>
Date: Wed, 17 Oct 2007 18:45:12 -0700

On Thu, 18 Oct 2007 11:25:42 +1000
Benjamin Herrenschmidt <[email-masked]> wrote:

> synchronize_irq needs at the very least a compiler barrier and a
> read barrier on SMP,

Why?

> but there are enough cases around where a
> write barrier is also needed and it's not a hot path so I prefer
> using a full smp_mb() here.
> 
> It will degrade to a compiler barrier on !SMP.
> 
> Signed-off-by: Benjamin Herrenschmidt <[email-masked]>
> ---
> 
> Index: linux-work/kernel/irq/manage.c
> ===================================================================
> --- linux-work.orig/kernel/irq/manage.c	2007-10-18 11:22:16.000000000 +1000
> +++ linux-work/kernel/irq/manage.c	2007-10-18 11:22:20.000000000 +1000
> @@ -33,6 +33,7 @@ void synchronize_irq(unsigned int irq)
>  	if (irq >= NR_IRQS)
>  		return;
>  
> +	smp_mb();
>  	while (desc->status & IRQ_INPROGRESS)
>  		cpu_relax();
>  }

Anyone reading this code is going to ask "wtf is that for".  It needs a
comment telling them.

mb() is the new lock_kernel().  Sigh.