netdev - Re: [PATCH net-next v4 1/2] driver core: auxiliary bus: show auxiliary device IRQs

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Message-ID: <2024051038-compare-canon-4161@gregkh>
Date: Fri, 10 May 2024 14:07:33 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Cc: Shay Drory <shayd@...dia.com>, netdev@...r.kernel.org,
	pabeni@...hat.com, davem@...emloft.net, kuba@...nel.org,
	edumazet@...gle.com, david.m.ertman@...el.com, rafael@...nel.org,
	ira.weiny@...el.com, linux-rdma@...r.kernel.org, leon@...nel.org,
	tariqt@...dia.com, Parav Pandit <parav@...dia.com>
Subject: Re: [PATCH net-next v4 1/2] driver core: auxiliary bus: show
 auxiliary device IRQs

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