linux-kernel - Re: [PATCH] Prevent nested interrupts when the IRQ stack is near overflowing v2

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Date:	Thu, 25 Mar 2010 12:16:17 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>, jesse.brandeburg@...el.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] Prevent nested interrupts when the IRQ stack is near
 overflowing v2

On Thu, 25 Mar 2010, Alan Cox wrote:

> > Which leads to the general question why we have that IRQF_DISABLED
> > shite at all. AFAICT the historical reason were IDE drivers, but we
> > grew other abusers like USB, SCSI and other crap which runs hard irq
> > handlers for hundreds of micro seconds in the worst case.
> 
> Anyone you've forgotten to offend ?

Hmm, not sure. Have not measured IRQ handler run times for quite a
while :)

> Pretty much the only 'core' driver today which enables IRQs in the irq
> handlers and needs it is the old IDE layer. There are also a couple of
> drivers which play games with disable/enable_irq in the IRQ paths for
> other reasons (lack of irq threads when written and a hardware model thats
> totally SMP unfriendly). 8390 is the obvious one here and it at least
> would be far far saner using threaded IRQs and normal locking with IRQs
> unmasked.

Right, but that's not the problem here. We talk about a (hopefully)
well written interrupt handler which runs for a very short
time. What's the point of running it with interrupts enabled ?
Nothing, we just run into stack overflow problems. So what's better:
an unreliable and ugly hackaround or just avoiding the possible stack
overflow in the first place ?

Thanks,

	tglx


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