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

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Message-ID: <5a3520fa-590f-48be-8594-de44ae4eb750@intel.com>
Date: Mon, 13 May 2024 10:33:18 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 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).