Date: Fri, 15 Dec 2023 11:13:45 -0800
From: David Wei <dw@...idwei.uk>
To: Jiri Pirko <jiri@...nulli.us>
Cc: Jakub Kicinski <kuba@...nel.org>, Sabrina Dubroca <sd@...asysnail.net>,
 netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v3 1/4] netdevsim: allow two netdevsim ports to
 be connected

On 2023-12-15 03:11, Jiri Pirko wrote:
> Thu, Dec 14, 2023 at 10:24:40PM CET, dw@...idwei.uk wrote:
>> Add a debugfs file in
>> /sys/kernel/debug/netdevsim/netdevsimN/ports/A/peer
>>
>> Writing "M B" to this file will link port A of netdevsim N with port B of
>> netdevsim M.
>>
>> Reading this file will return the linked netdevsim id and port, if any.
>>
>> Signed-off-by: David Wei <dw@...idwei.uk>
>> ---
>> drivers/net/netdevsim/bus.c       | 17 ++++++
>> drivers/net/netdevsim/dev.c       | 88 +++++++++++++++++++++++++++++++
>> drivers/net/netdevsim/netdev.c    |  6 +++
>> drivers/net/netdevsim/netdevsim.h |  3 ++
>> 4 files changed, 114 insertions(+)
>>
>> diff --git a/drivers/net/netdevsim/bus.c b/drivers/net/netdevsim/bus.c
>> index bcbc1e19edde..1ef95661a3f5 100644
>> --- a/drivers/net/netdevsim/bus.c
>> +++ b/drivers/net/netdevsim/bus.c
>> @@ -323,6 +323,23 @@ static struct device_driver nsim_driver = {
>> 	.owner		= THIS_MODULE,
>> };
>>
>> +struct nsim_bus_dev *nsim_bus_dev_get(unsigned int id)
> 
> This sounds definitelly incorrect. You should not need to touch bus.c
> code. It arranges the bus and devices on it. The fact that a device is
> probed or not is parallel to this.
> 
> I think you need to maintain a separate list/xarray of netdevsim devices
> probed by nsim_drv_probe()

There is a 1:1 relationship between bus devices (nsim_bus_dev) and nsim devices
(nsim_dev). Adding a separate list for nsim devices seemed redundant to me when
there is already a list for bus devices.

> 
> 
>> +{
>> +	struct nsim_bus_dev *nsim_bus_dev;
>> +
>> +	mutex_lock(&nsim_bus_dev_list_lock);
>> +	list_for_each_entry(nsim_bus_dev, &nsim_bus_dev_list, list) {
>> +		if (nsim_bus_dev->dev.id == id) {
>> +			get_device(&nsim_bus_dev->dev);
>> +			mutex_unlock(&nsim_bus_dev_list_lock);
>> +			return nsim_bus_dev;
>> +		}
>> +	}
>> +	mutex_unlock(&nsim_bus_dev_list_lock);
>> +
>> +	return NULL;
>> +}
>> +
>> int nsim_bus_init(void)
>> {
>> 	int err;
>> diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
>> index b4d3b9cde8bd..034145ba1861 100644
>> --- a/drivers/net/netdevsim/dev.c
>> +++ b/drivers/net/netdevsim/dev.c
>> @@ -388,6 +388,91 @@ static const struct file_operations nsim_dev_rate_parent_fops = {
>> 	.owner = THIS_MODULE,
>> };
>>
>> +static ssize_t nsim_dev_peer_read(struct file *file, char __user *data,
>> +				  size_t count, loff_t *ppos)
>> +{
>> +	struct nsim_dev_port *nsim_dev_port;
>> +	struct netdevsim *peer;
>> +	unsigned int id, port;
>> +	char buf[23];
>> +	ssize_t len;
>> +
>> +	nsim_dev_port = file->private_data;
>> +	rcu_read_lock();
>> +	peer = rcu_dereference(nsim_dev_port->ns->peer);
>> +	if (!peer) {
>> +		rcu_read_unlock();
>> +		return 0;
>> +	}
>> +
>> +	id = peer->nsim_bus_dev->dev.id;
>> +	port = peer->nsim_dev_port->port_index;
>> +	len = scnprintf(buf, sizeof(buf), "%u %u\n", id, port);
>> +
>> +	rcu_read_unlock();
>> +	return simple_read_from_buffer(data, count, ppos, buf, len);
>> +}
>> +
>> +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_bus_dev *peer_bus_dev;
>> +	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 for adding a peer is \"id port\" (uint uint)");
>> +		return -EINVAL;
>> +	}
>> +
>> +	/* invalid netdevsim id */
>> +	peer_bus_dev = nsim_bus_dev_get(id);
>> +	if (!peer_bus_dev)
>> +		return -EINVAL;
>> +
>> +	ret = -EINVAL;
>> +	/* cannot link to self */
>> +	nsim_dev_port = file->private_data;
>> +	if (nsim_dev_port->ns->nsim_bus_dev == peer_bus_dev &&
>> +	    nsim_dev_port->port_index == port)
>> +		goto out;
>> +
>> +	peer_dev = dev_get_drvdata(&peer_bus_dev->dev);
> 
> Again, no bus touching should be needed. (btw, this could be null is dev
> is not probed)

That's fair, I can do a null check.

> 
> 
>> +	list_for_each_entry(peer_dev_port, &peer_dev->port_list, list) {
>> +		if (peer_dev_port->port_index != port)
>> +			continue;
>> +		rcu_assign_pointer(nsim_dev_port->ns->peer, peer_dev_port->ns);
>> +		rcu_assign_pointer(peer_dev_port->ns->peer, nsim_dev_port->ns);
> 
> What is stopping another cpu from setting different peer for the same
> port here, making a mess?

Looking into RCU a bit more, you're right that it does not protect from
multiple writers. Would adding a lock (spinlock?) to nsim_dev and taking that
be sufficient here?

Or what if I took rtnl_lock()?

> 
> 
>> +		ret = count;
>> +		goto out;
>> +	}
>> +
>> +out:
>> +	put_device(&peer_bus_dev->dev);
>> +	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,
>> +	.owner = THIS_MODULE,
>> +};
>> +
>> static int nsim_dev_port_debugfs_init(struct nsim_dev *nsim_dev,
>> 				      struct nsim_dev_port *nsim_dev_port)
>> {
>> @@ -418,6 +503,9 @@ static int nsim_dev_port_debugfs_init(struct nsim_dev *nsim_dev,
>> 	}
>> 	debugfs_create_symlink("dev", nsim_dev_port->ddir, dev_link_name);
>>
>> +	debugfs_create_file("peer", 0600, nsim_dev_port->ddir,
>> +			    nsim_dev_port, &nsim_dev_peer_fops);
>> +
>> 	return 0;
>> }
>>
>> diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
>> index aecaf5f44374..e290c54b0e70 100644
>> --- a/drivers/net/netdevsim/netdev.c
>> +++ b/drivers/net/netdevsim/netdev.c
>> @@ -388,6 +388,7 @@ nsim_create(struct nsim_dev *nsim_dev, struct nsim_dev_port *nsim_dev_port)
>> 	ns->nsim_dev = nsim_dev;
>> 	ns->nsim_dev_port = nsim_dev_port;
>> 	ns->nsim_bus_dev = nsim_dev->nsim_bus_dev;
>> +	RCU_INIT_POINTER(ns->peer, NULL);
>> 	SET_NETDEV_DEV(dev, &ns->nsim_bus_dev->dev);
>> 	SET_NETDEV_DEVLINK_PORT(dev, &nsim_dev_port->devlink_port);
>> 	nsim_ethtool_init(ns);
>> @@ -407,9 +408,14 @@ nsim_create(struct nsim_dev *nsim_dev, struct nsim_dev_port *nsim_dev_port)
>> void nsim_destroy(struct netdevsim *ns)
>> {
>> 	struct net_device *dev = ns->netdev;
>> +	struct netdevsim *peer;
>>
>> 	rtnl_lock();
>> +	peer = rtnl_dereference(ns->peer);
>> +	RCU_INIT_POINTER(ns->peer, NULL);
>> 	unregister_netdevice(dev);
>> +	if (peer)
>> +		RCU_INIT_POINTER(peer->peer, NULL);
> 
> What is stopping the another CPU from setting this back to this "ns"?
> Or what is stopping another netdevsim port from setting this ns while
> going away?
> 
> Do you rely on RTNL_LOCK in any way (other then synchronize_net() in
> unlock())? If yes, looks wrong.
> 
> This ns->peer update locking looks very broken to me :/

As above, would a spinlock on nsim_dev or taking rtnl_lock() in
nsim_dev_peer_write() resolve this?

> 
> 
> 
> 
>> 	if (nsim_dev_port_is_pf(ns->nsim_dev_port)) {
>> 		nsim_macsec_teardown(ns);
>> 		nsim_ipsec_teardown(ns);
>> diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
>> index 028c825b86db..61ac3a80cf9a 100644
>> --- a/drivers/net/netdevsim/netdevsim.h
>> +++ b/drivers/net/netdevsim/netdevsim.h
>> @@ -125,6 +125,7 @@ struct netdevsim {
>> 	} udp_ports;
>>
>> 	struct nsim_ethtool ethtool;
>> +	struct netdevsim __rcu *peer;
>> };
>>
>> struct netdevsim *
>> @@ -415,5 +416,7 @@ struct nsim_bus_dev {
>> 	bool init;
>> };
>>
>> +struct nsim_bus_dev *nsim_bus_dev_get(unsigned int id);
>> +
>> int nsim_bus_init(void);
>> void nsim_bus_exit(void);
>> -- 
>> 2.39.3
>>

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