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

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Date:	Thu, 25 Mar 2010 17:27:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>, jesse.brandeburg@...el.com
Subject: Re: [PATCH] Prevent nested interrupts when the IRQ stack is near
 overflowing v2


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

[...]
>
> Now, it's also true that our IRQ infrastructure handlers _could_ be smarter, 
> and make the whole problem less likely to happen.
> 
> In particular, it's probably true that especially on modern hardware with 
> multiple cores, and especially when you do _not_ have irq sharing (which is 
> the common case these days for things like network drivers that can use 
> MSI), we really would be better off having the irq disabled over the whole 
> thing, and on some interrupt controllers it might even be worth it to do the 
> old optimization of not masking-and-acking, but just acking.

Yes.

> But see above. This is _not_ something that a driver can do any more. They 
> don't know whether the interrupt might end up being shared. Just blindly 
> setting IRAF_DISABLED in a driver is _not_ the answer. But being smarter in 
> the generic irq handler code might work.
> 
> And then, what we could do, is to mark the drivers that absolutely _must_ be 
> able to nest specially. Like the IDE driver when in PIO mode. Or maybe the 
> SCSI drivers, if they still depend on that timer interrupt happening while 
> they are busy.

I think the patch as posted solves a real problem, but also perpetuates a bad 
situation.

At minimum we should print a (one-time) warning that some badness occured. 
That would push us either in the direction of improving drivers, or towards 
improving the generic code.

Thanks,

	Ingo
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