Re: [PATCH] x86: Add check for number of available vectors before CPU down [v2]

[Prarit Bhargava <prarit@redhat.com>]

On 01/01/2014 09:41 PM, Chen, Gong wrote:
> On Tue, Dec 31, 2013 at 04:22:09PM -0500, Prarit Bhargava wrote:
>> Okay, how about,
>>                         if (irq_has_action(irq) && !irqd_is_per_cpu(data) &&
>>                             ((!cpumask_empty(&affinity_new)) &&
>>                               !cpumask_subset(&affinity_new, &online_new)) ||
>>                              cpumask_empty(&affinity_new))
>>                                 this_count++;
>>
> I think it is good but a little bit complicated. How about this:
> 
>         if (irq_has_action(irq) && !irqd_is_per_cpu(data) &&
>             /* add some commments to emphysize the importance of turn */
>             (cpumask_empty(&affinity_new) ||
>             !cpumask_subset(&affinity_new, &online_new)))

Yeah :)  I thought of that after I sent it. :)

> 
>> I tried this with the following examples and AFAICT I get the correct result:
>>
>> 1) affinity mask = online mask = 0xf.  CPU 3 (1000b) is down'd.
>>
>> this_count is not incremented.
>>
>> 2) affinity mask is a non-zero subset of the online mask (which IMO is
>> the "typical" case).  For example, affinity_mask = 0x9, online mask = 0xf.  CPU
>> 3 is again down'd.
>>
>> this_count is not incremented.
>>
>> 3) affinity_mask = 0x1, online mask = 0x3. (this is your example).  CPU
>> 1 is going down.
>>
>> this_count is incremented, as the resulting affinity mask will be 0.
>>
>> 4) affinity_mask = 0x0, online mask = 0x7.  CPU 1 is going down.
>>
>> this_count is incremented, as the affinity mask is 0.
>>
> The 4th scenario is very tricky. If you try to set affinity from user space,
> it will return failure because before kernel tried to change the affinity it
> will verify it:
> int __ioapic_set_affinity(...)
> {
> ...
>         if (!cpumask_intersects(mask, cpu_online_mask))
>                 return -EINVAL;
> ...
> }
> 
> So from this point of view, affinity can't be 0. But your patch is very
> special because you change it by hand:
>         cpu_clear(smp_processor_id(), affinity_new);
> 
> so it is reasonable. It makes me thinking a little bit more. In fixup_irqs
> we have similar logic but we don't protect it. Maybe it is because currently
> the scenario 4 can't happen because we stop it in advance. But who knows
> if one day we use it in other situation we will hit this subtle issue
> probably.
> 
> So, Prarit, I suggest you writing another patch to fix this potential issue
> for fixup_irqs. How would you think?

As you know Rui, I've been staring at that code wondering if it needs a fix.
I'd like to hear Gong Chen's thoughts about it too...

P.