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

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Message-ID: <4D08E391.2090205@siemens.com>
Date:	Wed, 15 Dec 2010 16:49:37 +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 16:41, Thomas Gleixner wrote:
> On Wed, 15 Dec 2010, Jan Kiszka wrote:
> 
>> 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.
> 
> Talking about headache. Your solution above does not prevent that
> scenario.
> 
>  CPU 0                  CPU 1
> 		
>  synchronize_irq();
> 		        hard irq comes in sees shared and unmasks

Nope, IRQ_ONESHOT is already cleared at that point.

>  clear IRQS_SHARED
> 			thread handler runs and sees !SHARED
> 
> Same scenario, just moved by a few lines :)

The same, just the other way around - and mostly harmless, I hope. :)

Jan

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