[PATCH] powerpc/xive: fix the fuzz value in xive_pick_irq_target()

[Michael Ellerman]

Benjamin Herrenschmidt <benh at kernel.crashing.org> writes:

> On Thu, 2017-08-03 at 09:45 +0200, Cédric Le Goater wrote:
>> On 08/02/2017 11:57 PM, Benjamin Herrenschmidt wrote:
>> > On Wed, 2017-08-02 at 18:43 +0200, Cédric Le Goater wrote:
>> > > If xive_find_target_in_mask() fails to find a cpu, the fuzz value used
>> > > in xive_pick_irq_target() is decremented and reused in the last
>> > > returning call to xive_find_target_in_mask(). This can result in such
>> > > WARNINGs if the initial fuzz value is zero :
>> > 
>> > Ah indeed ... would have worked better if "fuzz" had been unsigned.
>> 
>> but 'fuzz' is unsigned ! 
>
> Haha right.
>
>> With a -1, unsigned or not, the 'first' cpu  becomes out of range for
>> the calculation below :
>> 
>> 	/* Pick up a starting point CPU in the mask based on  fuzz */
>> 	num = cpumask_weight(mask);
>> 	first = fuzz % num;
>
> How can it ? fuzz % num should then return something that's
>
> 	0 <= first < num
>
> Regardless of the value of fuzz.

What if num is 0?

Which it would be in the fallback case, if the affinity mask is empty,
AFAICS.

cheers