Subject: Re: lockdep and threaded IRQs
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: David Miller <davem@davemloft.net>, dbrownell@users.sourceforge.net,
       tglx@linutronix.de, me@felipebalbi.com, linux-kernel@vger.kernel.org,
       linux-input@vger.kernel.org, felipe.balbi@nokia.com,
       dmitry.torokhov@gmail.com, sameo@openedhand.com
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Date: Tue, 03 Mar 2009 00:07:01 +0100
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On Mon, 2009-03-02 at 14:57 -0800, Andrew Morton wrote:
> On Mon, 02 Mar 2009 14:46:47 -0800 (PST)
> David Miller <davem@davemloft.net> wrote:
> 
> > From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Date: Mon, 02 Mar 2009 23:19:31 +0100
> > 
> > > I state that every !IRQF_DISABLED usage is a bug, either due to broken
> > > hardware or broken drivers.
> > 
> > We'll send you the bill to have everyone's hardware
> > replaced :-)
> 
> yes, but with what?
> 
> No matter how fast all our interrupt handlers are, running them with
> local interrupts disabled has to worsen the worst-case interrupt
> latency.
> 
> I don't see how removing !IRQF_DISABLED improves the kernel - in fact
> there's a latency argument for making !IRQF_DISABLED the default.

On preempt-rt all we do in the hardirq path is mask the interrupt line
and wake up a thread. That's the extreme end of low latency interrupts.

Arguably there is a middle way that works for !-rt.

However, striving to enable interrupts in all interrupt handlers is
asking for stack overruns. Interrupt nesting just isn't really helpful.

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