Re: [PATCH net v5 1/4] ax25: Use kernel universal linked list to implement ax25_dev_list

[Ratheesh Kannoth <rkannoth@marvell.com>]

On 2024-05-07 at 12:33:39, Duoming Zhou (duoming@zju.edu.cn) wrote:
> The origin ax25_dev_list implements its own single linked list,
> which is complicated and error-prone. For example, when deleting
> the node of ax25_dev_list in ax25_dev_device_down(), we have to
> operate on the head node and other nodes separately.
>
> This patch uses kernel universal linked list to replace original
> ax25_dev_list, which make the operation of ax25_dev_list easier.
> There are two points that need to notice:
>
> [1] We should add a check to judge whether the list is empty before
> INIT_LIST_HEAD in ax25_dev_device_up(), otherwise it will empty the
> list for each new ax25_dev added.
>
> [2] We should do "dev->ax25_ptr = ax25_dev;" and "dev->ax25_ptr = NULL;"
> while holding the spinlock, otherwise the ax25_dev_device_up() and
> ax25_dev_device_down() could race, we're not guaranteed to find a match
> ax25_dev in ax25_dev_device_down().
>
> Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
> -ax25_dev *ax25_dev_list;
> +static struct list_head ax25_dev_list;
>  DEFINE_SPINLOCK(ax25_dev_lock);
>
>  ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
> @@ -34,7 +35,7 @@ ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
>  	ax25_dev *ax25_dev, *res = NULL;
>
>  	spin_lock_bh(&ax25_dev_lock);
> -	for (ax25_dev = ax25_dev_list; ax25_dev != NULL; ax25_dev = ax25_dev->next)
> +	list_for_each_entry(ax25_dev, &ax25_dev_list, list)
>  		if (ax25cmp(addr, (const ax25_address *)ax25_dev->dev->dev_addr) == 0) {
>  			res = ax25_dev;
>  			ax25_dev_hold(ax25_dev);
> @@ -52,6 +53,9 @@ void ax25_dev_device_up(struct net_device *dev)
>  {
>  	ax25_dev *ax25_dev;
>
> +	/* Initialized the list for the first entry */
> +	if (!ax25_dev_list.next)
> +		INIT_LIST_HEAD(&ax25_dev_list);
if you define ax25_dev_list using 'static LIST_HEAD(ax25_dev_list)', you need this conditional check and
initialization ?

>  	ax25_dev = kzalloc(sizeof(*ax25_dev), GFP_KERNEL);
>  	if (!ax25_dev) {
>  		printk(KERN_ERR "AX.25: ax25_dev_device_up - out of memory\n");
> @@ -59,7 +63,6 @@ void ax25_dev_device_up(struct net_device *dev)
>  	}
>
>  	refcount_set(&ax25_dev->refcount, 1);
>