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

[Przemek Kitszel <przemyslaw.kitszel@intel.com>]

On 5/12/24 17:32, Jason Gunthorpe wrote:
> 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

What if I want to store some struct, potentially with need of some init
call (say, there will be a spinlock there)?

I believe the solution is to extend xarray so it will alloc the struct
(think flex array or user/priv data for "entry") and even init it
(so two new functions).