Re: [PATCH linux-next v2 1/2] irq: Add CPU mask affinity hint

[Peter P Waskiewicz Jr]

On Fri, 30 Apr 2010, Thomas Gleixner wrote:

> On Fri, 30 Apr 2010, Peter P Waskiewicz Jr wrote:
> > On Fri, 30 Apr 2010, Thomas Gleixner wrote:
> > > > +extern int irq_register_affinity_hint(unsigned int irq,
> > > > +                                      const struct cpumask *m);
> > >
> > > I think we can do with a single funtion irq_set_affinity_hint() and
> > > let the caller set the pointer to NULL.
> >
> > Ok, I've been running into some issues.  If CONFIG_CPUMASK_OFFSTACK is not
> > set, then cpumask_var_t structs are single-element arrays that cannot be
> > NULL'd out.  I'm pretty sure I need to keep the unregister part of the API.
> > Thoughts?
>
> extern int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m);
>
> So why should calling irq_set_affinity_hint(irqnr, NULL) not work ?

What was that you said about coffee and brain cells?  :-)

> > I just looked at the original show_affinity function, and it does not grab
> > desc->lock before copying mask out of desc.  Should I follow that model, or
> > should I fix that function to honor desc->lock?
>
> desc->affinity can only race against something changing the affinity
> bits, so that just might return some random data.
>
> In the hint case the irq could be shut down and the affinity hint
> could be freed while you are accessing it. Not a good idea :)

Good point.

Latest spin coming shortly.  Thanks for the quick feedback!

-PJ
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