linux-kernel - Re: [patch 1/6] hardirq: Make hardirq bits generic

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Date:	Thu, 7 Nov 2013 17:39:31 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
cc:	Andreas Schwab <schwab@...ux-m68k.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>, Peter Anvin <hpa@...or.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linux/m68k <linux-m68k@...r.kernel.org>
Subject: Re: [patch 1/6] hardirq: Make hardirq bits generic

On Thu, 7 Nov 2013, Geert Uytterhoeven wrote:

> Hi Thomas,
> 
> On Wed, Nov 6, 2013 at 6:23 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> Also note that the value of "nested" doesn't match the indentation level,
> >> which depends on my own bookkeeping using "nesting".
> >
> > Well, nested is just an indicator. It's not the nest level.
> 
> I know, the only thing that matters is whether it's zero or not.
> But it should always be zero if there's no nesting, and non-zero if there
> is, right?
> 
> So:
> 
> #   irq 13 nested 1024
> 
> nested should be 0 here.
> 
> #   irq 4 nested 0
> 
> ok
> 
> #     irq 13 nested 1024
> 
> ok (two extra spaces in front of "irq").
> 
> #     irq 4 nested 0
> 
> nested should be non-zero here.

Hmm. The softirq code reenables interrupts unconditionally. So when an
interrupt hits there SR on stack has the bits cleared. You could
verify that by checking in_serving_softirq() at the entry to
do_IRQ(). That could also explain the irq 4 nests in irq 4 issue. You
can't observe that on the original code as the softirq invocation and
therefor the interrupt enable happens outside of do_IRQ().

Though that does not explain the non nested case where nested is !=
0. But it looks like that irq 13 has a higher level than 4:

> #     irq 13 nested 1024
>
> ok (two extra spaces in front of "irq").

So it could actually be the following:

   irq X arrives, SR I2/1/0 is set to 4

   Now before we reach do_IRQ()

	irq 13 arrives and interrupts irq X as it has a higher level

   	Your nest accounting shows 0, but the SR says nested, which is
   	actually the correct state.

Is there an easy to setup/use emulator around on which I could try to
dig into that myself?

Thanks,

	tglx
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