linux-kernel - Re: [PATCH] irq: Remove unnecessary warning with affinity

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Message-ID: <5524D859.6010107@jp.fujitsu.com>
Date:	Wed, 8 Apr 2015 16:27:21 +0900
From:	Seiichi Ikarashi <s.ikarashi@...fujitsu.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	<linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] irq: Remove unnecessary warning with affinity_hint

Hi,

On 2015-04-08 15:28, Ingo Molnar wrote:
> 
> * Seiichi Ikarashi <s.ikarashi@...fujitsu.com> wrote:
> 
>> Hi,
>>
>> If you turn off a PCI device whose driver has set affinity_hint,
>> you will get warning message which does _not_ explain the reason
>> why it appeared from the user's point of view.
>>
>>   # echo 0 > /sys/bus/pci/slots/65/power
>>
>>   Apr 28 20:29:39 localhost kernel: ------------[ cut here ]------------
>>   Apr 28 20:29:39 localhost kernel: WARNING: at kernel/irq/manage.c:1002 __free_irq+0x22d/0x250() (Tainted: P           ---------------   )
>>   (snip)
>>
>> Users will misunderstand some problem has happened
>> even though he or she succeeded to turn off the device.
>> I suppose this warning was originally for a debug purpose
>> for driver developers and has incidentally been left.
>>
>> Just remove the warning is good and enough.
>>
>> Signed-off-by: Seiichi Ikarashi <s.ikarashi@...fujitsu.com>
>>
>> --- a/kernel/irq/manage.c
>> +++ b/kernel/irq/manage.c
>> @@ -1335,7 +1335,7 @@ static struct irqaction *__free_irq(unsi
>>  
>>  #ifdef CONFIG_SMP
>>  	/* make sure affinity_hint is cleaned up */
>> -	if (WARN_ON_ONCE(desc->affinity_hint))
>> +	if (desc->affinity_hint)
>>  		desc->affinity_hint = NULL;
> 
> Well, drivers that are using irq_set_affinity_hint() are expected to 
> call:
> 
> 	irq_set_affinity_hint(irq, NULL);
> 
> to clear the affinity mask, before releasing the irq. This warning 
> flags drivers that forgot to do that and which might thus leak a 
> dynamically allocated CPU mask (and/or other resources).

Calling irq_set_affinity_hint(irq, NULL) does not guarantee that
the driver does not forget to deallocate a dynamically allocated
CPU mask and/or other resources. But if calling it with NULL 2nd-arg
before releasing the irq is a virtual rule of using irq_set_affinity_hint()
interface, I understand it.

> 
> Feel free to turn the warning message into a more informative WARN() 
> that will blame the driver that triggered it, if the stack dump into 
> the driver wasn't a clue enough ...

Still, I do not know leaving the warning message is effective to
prevent drivers from potentially leaking resource... considering
a kind of cost-effectivenss. Business users (not developers) hate
such kind of messages for developers.

Thanks,
Seiichi

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