Re: [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when setting the hint

[Nitesh Lal]

On Tue, May 4, 2021 at 12:25 PM Jesse Brandeburg
<jesse.brandeburg@xxxxxxxxx> wrote:
>
> Robin Murphy wrote:
>
> > On 2021-05-01 03:18, Jesse Brandeburg wrote:
> > > It was pointed out by Nitesh that the original work I did in 2014
> > > to automatically set the interrupt affinity when requesting a
> > > mask is no longer necessary. The kernel has moved on and no
> > > longer has the original problem, BUT the original patch
> > > introduced a subtle bug when booting a system with reserved or
> > > excluded CPUs. Drivers calling this function with a mask value
> > > that included a CPU that was currently or in the future
> > > unavailable would generally not update the hint.
> > >
> > > I'm sure there are a million ways to solve this, but the simplest
> > > one is to just remove a little code that tries to force the
> > > affinity, as Nitesh has shown it fixes the bug and doesn't seem
> > > to introduce immediate side effects.
> >
> > Unfortunately, I think there are quite a few other drivers now relying
> > on this behaviour, since they are really using irq_set_affinity_hint()
> > as a proxy for irq_set_affinity(). Partly since the latter isn't
> > exported to modules, but also I have a vague memory of it being said
> > that it's nice to update the user-visible hint to match when the
> > affinity does have to be forced to something specific.
> >
> > Robin.
>
> Thanks for your feedback Robin, but there is definitely a bug here that
> is being exposed by this code. The fact that people are using this
> function means they're all exposed to this bug.
>
> Not sure if you saw, but this analysis from Nitesh explains what
> happened chronologically to the kernel w.r.t this code, it's a useful
> analysis! [1]
>
> I'd add in addition that irqbalance daemon *stopped* paying attention
> to hints quite a while ago, so I'm not quite sure what purpose they
> serve.
>
> [1]
> https://lore.kernel.org/lkml/CAFki+Lm0W_brLu31epqD3gAV+WNKOJfVDfX2M8ZM__aj3nv9uA@xxxxxxxxxxxxxx/
>

Wanted to follow up to see if there are any more objections or even
suggestions to take this forward?

-- 
Thanks
Nitesh