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

Thomas Gleixner
2010-04-28T16:45:36Z

B1;2005;0cPeter,

On Tue, 27 Apr 2010, Peter P Waskiewicz Jr wrote:
> On Tue, 27 Apr 2010, Thomas Gleixner wrote:
> > On Sun, 18 Apr 2010, Peter P Waskiewicz Jr wrote:
> > > +/**
> > > + * struct irqaffinityhint - per interrupt affinity helper
> > > + * @callback: device driver callback function
> > > + * @dev: reference for the affected device
> > > + * @irq: interrupt number
> > > + */
> > > +struct irqaffinityhint {
> > > + irq_affinity_hint_t callback;
> > > + void *dev;
> > > + int irq;
> > > +};
> >
> > Why do you need that extra data structure ? The device and the irq
> > number are known, so all you need is the callback itself. So no need
> > for allocating memory ....
>
> When I register the function callback with the interrupt layer, I need to
> know what device structures to reference back in the driver. In other words,
> if I call into an underlying driver with just an interrupt number, then I
> have no way at getting at the dev structures (netdevice for me, plus my
> private adapter structures), unless I declare them globally (yuck).

Grr, I knew that I missed something. That'll teach me to review
patches before the coffee has reached my brain cells :)

> I had a different approach before this one where I assumed the device from
> the irq handler callback was safe to use for the device in this new callback.
> I didn't feel really great about that, since it's an implicit assumption that
> could cause things to go sideways really quickly.
>
> Let me know what you think either way. I'm certainly willing to make a
> change, I just don't know at this point what's the safest approach from what
> I currently have.

So you need a reference to your device, so what about the following:

struct irq_affinity_hint;

struct irq_affinity_hint {
unsigned int (*callback)(unsigned int irq, struct irq_affinity_hint *hint,
cpumask_var_t *mask);
}

Now you embed that struct into your device private data structure and
you get the reference to it back in the callback function. No extra
kmalloc/kfree, less code.

One other thing I noticed, but forgot to comment on:

> +static int irq_affinity_hint_proc_show(struct seq_file *m, void *v)
> +{
> + struct irq_desc *desc = irq_to_desc((long)m->private);
> + struct cpumask mask;
> + unsigned int ret = 0;

Why do we return 0, when there is no callback and no hint available ?

> +

We don't want to have cpumask enforced on stack. Please make that:

cpumask_var_t mask;

if (!alloc_cpumask_var(&mask, GFP_KERNEL))
return -ENOMEM;

> + if (desc->hint && desc->hint->callback) {

The access to desc-> needs to be protected with
desc->lock. Otherwise you might race with a callback unregister.

> + ret = desc->hint->callback(&mask, (long)m->private,
> + desc->hint->dev);
> + if (!ret)
> + seq_cpumask(m, &mask);
> + }
> +
> + seq_putc(m, '\n');
> + return ret;
> +}

Thanks,

tglx