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SubjectRe: [PATCH] synchronize_irq needs a barrier
On Thu, 18 Oct 2007 11:25:42 +1000
Benjamin Herrenschmidt <benh@kernel.crashing.org> 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 <benh@kernel.crashing.org>
> ---
>
> 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.
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