linux-kernel - Re: [PATCH v3 2/4] genirq: Inform handler about line sharing state

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Message-ID: <alpine.LFD.2.00.1012151357030.12146@localhost6.localdomain6>
Date:	Wed, 15 Dec 2010 14:04:13 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jan Kiszka <jan.kiszka@....de>
cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>,
	Tom Lyon <pugs@...co.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>
Subject: Re: [PATCH v3 2/4] genirq: Inform handler about line sharing state

On Wed, 15 Dec 2010, Jan Kiszka wrote:
> Am 14.12.2010 21:54, Thomas Gleixner wrote:
> > On Mon, 13 Dec 2010, Jan Kiszka wrote:
> >> @@ -943,6 +950,9 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id)
> >>  	/* Make sure it's not being used on another CPU: */
> >>  	synchronize_irq(irq);
> >>  
> >> +	if (single_handler)
> >> +		desc->irq_data.drv_status &= ~IRQS_SHARED;
> >> +
> > 
> > What's the reason to clear this flag outside of the desc->lock held
> > region.
> 
> We need to synchronize the irq first before clearing the flag.
> 
> The problematic scenario behind this: An IRQ started in shared mode,
> this the line was unmasked after the hardirq. Now we clear IRQS_SHARED
> before calling into the threaded handler. And that handler may now think
> that the line is still masked as IRQS_SHARED is set.

That should read "not set" I guess. Hmm, needs more thoughts :(
 
> > I need this status for other purposes as well, where I
> > definitely need serialization.
> 
> Well, two options: wrap all bit manipulations with desc->lock
> acquisition/release or turn drv_status into an atomic. I don't know what
> your plans with drv_status are, so...

Some bits for irq migration and other stuff, which allows us to avoid
fiddling with irqdesc in the drivers.

Thanks,

	tglx
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