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

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Message-ID: <4D0A75E3.3090900@web.de>
Date:	Thu, 16 Dec 2010 21:26:11 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Thomas Gleixner <tglx@...utronix.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

Am 16.12.2010 14:13, Thomas Gleixner wrote:
> On Mon, 13 Dec 2010, Jan Kiszka wrote:
>> +	if (old_action && (old_action->flags & IRQF_ADAPTIVE) &&
>> +	    !(desc->irq_data.drv_status & IRQS_SHARED)) {
>> +		/*
>> +		 * Signal the old handler that is has to switch to shareable
>> +		 * handling mode. Disable the line to avoid any conflict with
>> +		 * a real IRQ.
>> +		 */
>> +		disable_irq(irq);
> 
> This is weird, really. I thought you wanted to avoid waiting for the
> threaded handler to finish if it's on the fly. So this should be
> disable_irq_nosync() or did you change your mind ?

No, I did not. I wanted to avoid that we set MAKE_SHAREABLE while there
might be another IRQ in flight. The handler that is called due to a real
IRQ might misinterpret MAKE_SHAREABLE as "there is no real event" and
perform the wrong steps (at least the current implementation for KVM would).

However, I will rebase my patch over your series now and try to re-think
this. The question is what could go wrong if we do not guarantee that
MAKE_SHAREABLE and ordinary IRQ will always be distinguishable. If there
is really nothing, specifically for the KVM scenario, we could even drop
the disable/enable_irq. That would be also be nicer when thinking about
potential delays of the already registered handler during this
transitional phase.

Jan

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