Re: [PATCH net-next v2 2/3] net: dsa: add Arrow SpeedChips XRS700x driver

[Vladimir Oltean <olteanv@gmail.com>]

On Sat, Nov 28, 2020 at 01:39:16AM +0200, Vladimir Oltean wrote:
> On Sat, Nov 28, 2020 at 12:30:48AM +0100, Andrew Lunn wrote:
> > > If there is a better alternative I'm all ears but having /proc and
> > > ifconfig return zeros for error counts while ip link doesn't will lead
> > > to too much confusion IMO. While delayed update of stats is a fact of
> > > life for _years_ now (hence it was backed into the ethtool -C API).
> >
> > How about dev_seq_start() issues a netdev notifier chain event, asking
> > devices which care to update their cached rtnl_link_stats64 counters.
> > They can decide if their cache is too old, and do a blocking read for
> > new values.
> >
> > Once the notifier has completed, dev_seq_start() can then
> > rcu_read_lock() and do the actual collection of stats from the drivers
> > non-blocking.
>
> That sounds smart. I can try to prototype that and see how well it
> works, or do you want to?

The situation is like this:

static int call_netdevice_notifiers_info(unsigned long val,
					 struct netdev_notifier_info *info);

expects a non-NULL info->dev argument.

To get a net device you need to call:

#define for_each_netdev(net, d)		\
		list_for_each_entry(d, &(net)->dev_base_head, dev_list)

which has the following protection rules:

/*
 * The @dev_base_head list is protected by @dev_base_lock and the rtnl
 * semaphore.
 *
 * Pure readers hold dev_base_lock for reading, or rcu_read_lock()
 *
 * Writers must hold the rtnl semaphore while they loop through the
 * dev_base_head list, and hold dev_base_lock for writing when they do the
 * actual updates.  This allows pure readers to access the list even
 * while a writer is preparing to update it.
 *
 * To put it another way, dev_base_lock is held for writing only to
 * protect against pure readers; the rtnl semaphore provides the
 * protection against other writers.
 *
 * See, for example usages, register_netdevice() and
 * unregister_netdevice(), which must be called with the rtnl
 * semaphore held.
 */

This means, as far as I understand, 2 things:
1. call_netdevice_notifiers_info doesn't help, since our problem is the
   same
2. I think that holding the RTNL should also be a valid way to iterate
   through the net devices in the current netns, and doing just that
   could be the simplest way out. It certainly worked when I tried it.
   But those could also be famous last words...