Subject: Re: [PATCH net-next v5 2/5] netdevsim: allow two netdevsim ports to be connected

From: Jakub Kicinski <[email-masked]>
Date: Wed, 3 Jan 2024 17:39:28 -0800

On Wed, 27 Dec 2023 17:46:30 -0800 David Wei wrote:
> +static ssize_t nsim_dev_peer_write(struct file *file,
> +				   const char __user *data,
> +				   size_t count, loff_t *ppos)
> +{
> +	struct nsim_dev_port *nsim_dev_port, *peer_dev_port;
> +	struct nsim_dev *peer_dev;
> +	unsigned int id, port;
> +	char buf[22];
> +	ssize_t ret;
> +
> +	if (count >= sizeof(buf))
> +		return -ENOSPC;
> +
> +	ret = copy_from_user(buf, data, count);
> +	if (ret)
> +		return -EFAULT;
> +	buf[count] = '\0';
> +
> +	ret = sscanf(buf, "%u %u", &id, &port);
> +	if (ret != 2) {
> +		pr_err("Format is peer netdevsim \"id port\" (uint uint)\n");

netif_err() or dev_err() ? Granted the rest of the file seems to use
pr_err(), but I'm not sure why...

> +		return -EINVAL;
> +	}

Could you put a sleep() here and test removing the device while some
thread is stuck here? I don't recall exactly but I thought debugfs
remove waits for concurrent reads and writes which could be problematic
given we take all the locks under the sun here..

> +	ret = -EINVAL;
> +	mutex_lock(&nsim_dev_list_lock);
> +	peer_dev = nsim_dev_find_by_id(id);
> +	if (!peer_dev) {
> +		pr_err("Peer netdevsim %u does not exist\n", id);
> +		goto out_mutex;
> +	}
> +
> +	devl_lock(priv_to_devlink(peer_dev));
> +	rtnl_lock();
> +	nsim_dev_port = file->private_data;
> +	peer_dev_port = __nsim_dev_port_lookup(peer_dev, NSIM_DEV_PORT_TYPE_PF,
> +					       port);
> +	if (!peer_dev_port) {
> +		pr_err("Peer netdevsim %u port %u does not exist\n", id, port);
> +		goto out_devl;
> +	}
> +
> +	if (nsim_dev_port == peer_dev_port) {
> +		pr_err("Cannot link netdevsim to itself\n");
> +		goto out_devl;
> +	}
> +
> +	rcu_assign_pointer(nsim_dev_port->ns->peer, peer_dev_port->ns);
> +	rcu_assign_pointer(peer_dev_port->ns->peer, nsim_dev_port->ns);
> +	ret = count;
> +
> +out_devl:

out_unlock_rtnl

> +	rtnl_unlock();
> +	devl_unlock(priv_to_devlink(peer_dev));
> +out_mutex:

out_unlock_dev_list

> +	mutex_unlock(&nsim_dev_list_lock);
> +
> +	return ret;
> +}
> +
> +static const struct file_operations nsim_dev_peer_fops = {
> +	.open = simple_open,
> +	.read = nsim_dev_peer_read,
> +	.write = nsim_dev_peer_write,
> +	.llseek = generic_file_llseek,

You don't support seek, you want some form of no_seek here.

> +	.owner = THIS_MODULE,
> +};