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

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Date:	Wed, 8 Apr 2015 09:39:38 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Seiichi Ikarashi <s.ikarashi@...fujitsu.com>
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


* Seiichi Ikarashi <s.ikarashi@...fujitsu.com> wrote:

> 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. [...]

I said 'might leak', not 'guaranteed to leak'.

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.

Thanks,

	Ingo
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