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

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Message-ID: <4D08CE44.6000407@siemens.com>
Date:	Wed, 15 Dec 2010 15:18:44 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Jan Kiszka <jan.kiszka@....de>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <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>
Subject: Re: [PATCH v3 2/4] genirq: Inform handler about line sharing state

Am 15.12.2010 14:04, Thomas Gleixner wrote:
> 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.

Can't remember who wrote this, but that guy might have been too tired
for clear sentences: Yes, of course, we could run into troubles, if
IRQS_SHARED was _not_ set while the IRQ line is unmasked between hard
and threaded handler.

> Hmm, needs more thoughts :(

Be warned, might be painful.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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