linux-kernel - Re: PCI: MSI interrupts masked using prohibited method

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Date:	Thu, 17 Jul 2008 19:04:05 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Matthew Wilcox <matthew@....cx>
cc:	David Vrabel <david.vrabel@....com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: PCI: MSI interrupts masked using prohibited method

On Thu, 17 Jul 2008, Matthew Wilcox wrote:

> On Thu, Jul 17, 2008 at 05:58:26PM +0200, Thomas Gleixner wrote:
> > > This is a good thought, let's follow it through.  What if we simply make
> > > ->mask a no-op for devices which don't support mask bits?
> > 
> > Yep. You can also use fasteoi_handler, which just calls ->eoi() after
> > the handler.
> 
> I think that exposes us to a race.
> 
> CPU takes the first interrupt, calls handle_fasteoi_irq().  That
> calls handle_IRQ_event() which calls the device's interrupt handler.
> Interrupt handler reads status register to determine what to do next.
> Device generates second interrupt and changes status register.  Second
> interrupt is never delivered because the ->eoi hasn't been called yet.

Yeah, I know. The question is how the hardware works; there is fasteoi
capable hardware around (not on x86) which works with edge type
interrupts.
 
> I plan to keep using the edge handler which solves this race by
> calling mask_ack().  For MSIs without mask bits, it will do nothing.

Ah, there are ones w/o a mask bit. That detail slipped through.

Thanks,

	tglx
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