Re: [PATCH] irq: Remove unnecessary warning with affinity_hint

Seiichi Ikarashi
2015-04-08T08:06:47Z

On 2015-04-08 16:39, Ingo Molnar wrote:
>
> * Seiichi Ikarashi <s.ikarashi@jp.fujitsu.com> wrote:
>
>> Hi,
>>
>> On 2015-04-08 15:28, Ingo Molnar wrote:
>>>
>>> * Seiichi Ikarashi <s.ikarashi@jp.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@jp.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. [...]
>
> I said 'might leak', not 'guaranteed to leak'.

Yes, I know.
I wrote it because I was not sure about the primary purpose of
showing the warning message.

>
> Calling irq_set_affinity_hint(irq, NULL) is the way this kernel API is
> specified to be used. Forgetting to do it is a kernel driver bug and
> triggers a warning message from the kernel's IRQ subsystem.
>
>> [...] 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.
>
> it's a warning message pointing out a kernel bug: that
> irq_set_affinity_hint(irq, NULL) was not called properly.
>
> Messages pointing out kernel bugs should be fixed, not hidden.

OK, the conclusion is that a kernel bug on using irq_set_affinity_hint().

Thanks, Ingo.

Seiichi