On Fri, May 10, 2024 at 02:54:49PM +0200, Przemek Kitszel wrote:
> > > +static ssize_t auxiliary_irq_mode_show(struct device *dev,
> > > + struct device_attribute *attr, char *buf)
> > > +{
> > > + struct auxiliary_irq_info *info =
> > > + container_of(attr, struct auxiliary_irq_info, sysfs_attr);
> > > +
> > > + if (refcount_read(xa_load(&irqs, info->irq)) > 1)
> >
> > refcount combined with xa? That feels wrong, why is refcount used for
> > this at all?
>
> Not long ago I commented on similar usage for ice driver,
> ~"since you are locking anyway this could be a plain counter",
> and author replied
> ~"additional semantics (like saturation) of refcount make me feel warm
> and fuzzy" (sorry if misquoting too much).
> That convinced me back then, so I kept quiet about that here.
But why is this being incremented / decremented at all? What is that
for?
> The "use least powerful option" rule of thumb is perhaps more important.
Yes, but use a refcount properly if needed, I can't figure out why a
refcount is needed here at all, which is not a good sign.
> > > + refcount_set(new_ref, 1);
> > > + ref = __xa_cmpxchg(&irqs, irq, NULL, new_ref, GFP_KERNEL);
> > > + if (ref) {
> > > + kfree(new_ref);
> > > + if (xa_is_err(ref)) {
> > > + ret = xa_err(ref);
> > > + goto out;
> > > + }
> > > +
> > > + /* Another thread beat us to creating the enrtry. */
> > > + refcount_inc(ref);
> >
> > How can that happen? Why not just use a normal simple lock for all of
> > this so you don't have to mess with refcounts at all? This is not
> > performance-relevent code at all, but yet with a refcount you cause
> > almost the same issues that a normal lock would have, plus the increased
> > complexity of all of the surrounding code (like this, and the crazy
> > __xa_cmpxchg() call)
> >
> > Make this simple please.
>
> I find current API of xarray not ideal for this use case, and would like
> to fix it, but let me write a proper RFC to don't derail (or slow down)
> this series.
Why do you need to use an xarray here at all? Why isn't this just tied
directly to the aux device instead?
thanks,
greg k-h
