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

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Message-ID: <ZkDg8Aj/TdOqFwqf@ziepe.ca>
Date: Sun, 12 May 2024 12:32:00 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Cc: Greg KH <gregkh@...uxfoundation.org>, 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:
> > > +	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.

I think xarray can do this just fine already??

xa_lock(&irqs);
used = xa_to_value(xa_load(&irqs, irq));
used++;
ret = xa_store(&irqs, irq, xa_mk_value(used));
xa_unlock(&irqs);

And you can safely read the value using the typical xa_load RCU locking.

Jason