netdev - Re: [patch net-next v2 1/9] Introduce devlink infrastructure

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Date:	Wed, 24 Feb 2016 08:19:58 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Jarod Wilson <jarod@...hat.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, idosch@...lanox.com,
	eladr@...lanox.com, yotamg@...lanox.com, ogerlitz@...lanox.com,
	yishaih@...lanox.com, dledford@...hat.com, sean.hefty@...el.com,
	hal.rosenstock@...il.com, eugenia@...lanox.com,
	roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
	hadarh@...lanox.com, jhs@...atatu.com, john.fastabend@...il.com,
	jeffrey.t.kirsher@...el.com, brouer@...hat.com, ivecera@...hat.com,
	rami.rosen@...el.com, hannes@...essinduktion.org,
	gospo@...ulusnetworks.com
Subject: Re: [patch net-next v2 1/9] Introduce devlink infrastructure

Tue, Feb 23, 2016 at 10:19:48PM CET, jarod@...hat.com wrote:
>On Tue, Feb 23, 2016 at 04:51:26PM +0100, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@...lanox.com>
>> 
>> Introduce devlink infrastructure for drivers to register and expose to
>> userspace via generic Netlink interface.
>> 
>> There are two basic objects defined:
>> devlink - one instance for every "parent device", for example switch ASIC
>> devlink port - one instance for every physical port of the device.
>> 
>> This initial portion implements basic get/dump of objects to userspace.
>> Also, port splitter and port type setting is implemented.
>...
>> +/**
>> + *	devlink_alloc - Allocate new devlink instance resources
>> + *
>> + *	@ops: ops
>> + *	@priv_size: size of user private data
>> + *
>> + *	Allocate new devlink instance resources, including devlink index
>> + *	and name.
>> + */
>> +struct devlink *devlink_alloc(const struct devlink_ops *ops, size_t priv_size)
>> +{
>> +	static atomic_t dev_counter = ATOMIC_INIT(0);
>> +	struct devlink *devlink;
>> +
>> +	devlink = kzalloc(sizeof(*devlink) + priv_size, GFP_KERNEL);
>> +	if (!devlink)
>> +		return NULL;
>> +	devlink->ops = ops;
>> +	devlink_net_set(devlink, &init_net);
>> +
>> +different_name:
>> +	devlink->index = atomic_inc_return(&dev_counter);
>> +	if (devlink->index < 0) {
>> +		/* wrapped */
>> +		atomic_dec(&dev_counter);
>> +		kfree(devlink);
>> +		return NULL;
>> +	}
>> +	/* atomic_inc_return makes it start at 1, make it start at 0 */
>> +	devlink->index--;
>> +
>> +	dev_set_name(&devlink->dev, DEVLINK_GENL_NAME "%d", devlink->index);
>> +	if (devlink_name_exists(devlink_net(devlink), devlink_name(devlink)))
>> +		goto different_name;
>> +
>> +	INIT_LIST_HEAD(&devlink->port_list);
>> +	device_initialize(&devlink->dev);
>> +	devlink->dev.class = &devlink_class;
>> +	devlink->dev.platform_data = devlink;
>> +	return devlink;
>> +}
>> +EXPORT_SYMBOL_GPL(devlink_alloc);
>
>The increment then decrement bit feels... Clunky and a little confusing.
>Why not just atomic_read(&dev_counter) before different_name:, then an
>atomic_inc_return(&dev_counter) under the if clause before the goto, and
>eliminate the devlink->index-- bit?

If I split it it would not be atomic op anymore. But the
devlink->index-- is just to adjust the start, nothing else.
This is copied from nl80211 code.