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Subject: Re: [PATCH] i386: Fix softirq accounting with 4K stacks
From: Arjan van de Ven <arjan@infradead.org>
To: Mike Galbraith <efault@gmx.de>
Cc: =?ISO-8859-1?Q?Bj=F6rn?= Steinbrink <B.Steinbrink@gmx.de>,
       linux-kernel@vger.kernel.org, danial_thom@yahoo.com,
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Date: Sun, 25 Jun 2006 19:43:17 +0200
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On Sun, 2006-06-25 at 19:44 +0200, Mike Galbraith wrote:
> On Sun, 2006-06-25 at 16:24 +0200, Björn Steinbrink wrote:
> 
> > Still no idea why your "fix" works, but the following patch also fixes
> > the problem and is at least a little more like the RightThing.
> 
> Yeah.  I don't know about you, but I fully intend to blatantly ignore
> that 'why' ;-)

the why is relatively easy ;)

since the "is a softirq executing" bit is on the stack, and each context
(user, soft and hard irq) has their own stack, it's not automatic that
the hardirq stack gets the softirq-executing flag... which your patch
fixes.

NMI's and apic irqs generally don't go via the normal irq path and thus
don't do a stack switch... so they don't lose the bit (for accounting
purposes)