linux-kernel - Re: [RFC][PATCH] irq: remove IRQF

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Date:	Fri, 06 Mar 2009 11:24:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH] irq: remove IRQF_DISABLED

On Fri, 2009-03-06 at 20:59 +1100, Benjamin Herrenschmidt wrote:
> > If you have distinct interrupt priorities, you can
> > 
> > 1) provide an interrupt stack for each priority
> > 2) mask all lower priorities when handling one
> > 
> > Would that not work?
> 
> The PIC does that already. IE. it will only interrupt again before
> ->eoi() for an interrupt of a higher priority. But by using
> IRQF_DISABLED, you mask interrupts in the core, and thus effectively
> completely prevents the whole thing.
> 
> > The problems with enabling irqs in hardirq handlers are that you get
> > unlimited irq nesting, which is bad for your stack, furthermore, somehow
> > people thing it makes things 'faster' because the irq-off latency goes
> > down.
> 
> No, you don't get unlimited IRQ nesting, at least not on sane archs with
> a decent PIC that does things like what I described above :-)

Right, welcome to x86 ;-)

Sounds to me we need to extend genirq a bit... Thomas?

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