[PATCH] arm64: do not force irq affinity setting

[Prashant Gaikwad]

On Thu, 2014-06-26 at 18:41 +0530, Will Deacon wrote:
> On Thu, Jun 26, 2014 at 01:00:24PM +0100, Prashant Gaikwad wrote:
> > On Thu, 2014-06-26 at 15:50 +0530, Will Deacon wrote:
> > > On Thu, Jun 26, 2014 at 07:49:55AM +0100, Prashant Gaikwad wrote:
> > > > Unconditional copying cpu_online_mask to affinity
> > > > may result in migrating affinity to wrong CPU.
> > > 
> > > We have a bug, but I don't follow your reasoning.
> > > 
> > > > For example, IRQ 5 affinity mask contains CPU 4-7,
> > > 
> > > Ok, so d->affinity is 0xf0...
> > > 
> > > > it was affined to CPU4 and CPU 0-7 are online.
> > > 
> > > ...and cpu_online_mask is 0xff.
> > > 
> > > > Now if we hot-unplug CPU4 then with current
> > > > implementation affinity mask will contain
> > > > CPU 0-3,5-7 and IRQ 5 will be affined to CPU0.
> > > 
> > > cpumask_any_and(affinity, cpu_online_mask) will give return < nr_cpu_ids
> > > since there is an intersection of 0xf0. That means ret is false.
> > > 
> > > The bug is that we then do affinity = cpu_online_mask; unconditionally,
> > > but we *won't* do the cpumask_copy, since ret is false.
> > > 
> > 
> > We do not copy but the affinity mask passed to irq_set_affinity function
> > is nothing but cpu_online_mask. So in GIC it will set affinity to CPU0.
> 
> Exactly, but your proposed patch changed more than that.
> 

I am changing the force flag to false. That is because after I fix this
behavior we have another bug where the IRQ affinity is set to offline
CPU.

When cpumask_any_and(affinity, cpu_online_mask) return < nr_cpu_ids we
pass the affinity mask as it is which contains the offline CPU too and
if force flag is true then GIC driver skips online CPU check. If CPU0 is
going down then the affinity mask will have CPU0 and GIC driver will
keep the affinity to CPU0.

Changing force flag to false ensures that GIC driver checks for online
CPU.

> > > You can fix this by simply bringing the arm64 code into line with the arm
> > > code, which begs the question as to why this has to exist in the arch/
> > > backend at all!
> > 
> > Where can we move this code?
> 
> kernel/irq/migration.c?
> 
> Will